There’s https://github.com/crystal-lang/crystal/wiki/Crystal-for-Rubyists, but I think nobody keeps that up-to-date.
In Ruby lambda is just a Proc that behaves a bit differently in some aspects. In Crystal we never had the need for those distinctions, so there’s just Proc. Also Ruby does actually provide a quite similar syntax to define a lambda proc, the “stabby lambda”: ->(x) { x + 1 }.
You’re right. It hasn’t been updated for either Ruby or Crystal.
I don’t want to change this thread to about documentation (keep it about Rosetta Code) so I’ll make a last comment here about Crystal documentation.
I spent almost 3 hrs before giving up because of deficiencies in website ref docs. Ruby’s std docs, in contrast, are more detailed, and more people write about its features/nuances.
I’ve been encouraging the devs to TAKE THE TIME to at least write regular blog entries on the website on small issues like this, to document these things in at least one location for users.
For example, I’m really impressed with the quality and breadth of the Sidef language and documentation, which I hadn’t even heard of until Rosetta Code exposure.
Again, I don’t want to hijack this thread away from Rosetta Code. If there’s an existing one dedicated to documentation I can contribute these point there, but good documentation is essential to enable users to come up to speed fast. Lastly, here are some helpful resources.
Tu 2020/4/21
Jacobi symbol
https://rosettacode.org/wiki/Jacobi_symbol#Crystal
Wed 2020/4/22
Greatest subsequential sum
https://rosettacode.org/wiki/Greatest_subsequential_sum#Crystal
Th 2020/4/23
SHA-1
https://rosettacode.org/wiki/SHA-1#Crystal
SHA-256
https://rosettacode.org/wiki/SHA-256#Crystal
Factors of an integer
https://rosettacode.org/wiki/Factors_of_an_integer#Crystal
Self describing numbers
https://rosettacode.org/wiki/Self-describing_numbers#Crystal
Mon 20202/4/27
Greatest element of a list
https://rosettacode.org/wiki/Greatest_element_of_a_list#Crystal
Guess the number
https://rosettacode.org/wiki/Guess_the_number#Crystal
Grayscale image
https://rosettacode.org/wiki/Grayscale_image#Crystal
Guess the number/With feedback
https://rosettacode.org/wiki/Guess_the_number/With_feedback#Crystal
Vector products
https://rosettacode.org/wiki/Vector_products#Crystal
Dot product
https://rosettacode.org/wiki/Dot_product#Crystal
Rate counter
https://rosettacode.org/wiki/Rate_counter#Crystal
Tu 2020/4/28
Quaternion type
https://rosettacode.org/wiki/Quaternion_type#Crystal
Th 2020/4/30
Largest number divisible by its digits
https://rosettacode.org/wiki/Largest_number_divisible_by_its_digits#Crystal
Integer sequence
https://rosettacode.org/wiki/Integer_sequence#Crystal
Gapful numbers
https://rosettacode.org/wiki/Gapful_numbers#Crystal
Tu 2020/5/5
Look and say sequence
https://rosettacode.org/wiki/Look-and-say_sequence#Crystal
Long multiplication
https://rosettacode.org/wiki/Long_multiplication#Crystal
Th 2020/5/7
Multifactorial
https://rosettacode.org/wiki/Multifactorial#Crystal
Mind boggling card trick
https://rosettacode.org/wiki/Mind_boggling_card_trick#Crystal
75 Tasks Done!! 
Fri 2020/5/8
Hamming numbers
https://rosettacode.org/wiki/Hamming_numbers#Crystal
Fri 2020/05/08
Base64 decode data
https://rosettacode.org/wiki/Base64_decode_data#Crystal
Welcome to the club @isfalcantara! 
FYI: can you edit your post and put the date at the top, like the others, to document its date. I’ve included your entry in the master list I’m keeping. When it hits 100 I’ll post the complete list (with dates and authors) here again.
2020/05/10
Read a file character by character/UTF8
http://www.rosettacode.org/wiki/Read_a_file_character_by_character/UTF8#Crystal
Read entire file
http://www.rosettacode.org/wiki/Read_entire_file#Crystal
Cool! The default encoding is UTF-8 so there’s no need to explicitly specify it (it’s just ignored).
Tu 2020/5/12
Humble numbers
http://rosettacode.org/wiki/Humble_numbers#Crystal
Wed 2020/5/13
Added orders of magnitude faster version.
Th 2020/5/14
Mon 2020/5/18
Palindromic gapful numbers
https://rosettacode.org/wiki/Palindromic_gapful_numbers#Crystal
Wed 2020/5/20
Update: Added slightly stylistically modified Object Oriented version done by @jhass.
Semordnilap
http://rosettacode.org/wiki/Semordnilap#Crystal