Correct lucky_numbers link and listed filename

[juangallostra]

Description of the issue/feature this PR addresses

Instead of being redirected to the implementation, clicking over lucky_number.cpp results in a 404 error being raised.

Also, the implementation is in a file named lucky_numbers.cpp and not lucky_number.cpp

Current behavior before PR

1. 404 error is raised when clicking over the cpp implementation of lucky numbers
2. Listed filename with the implementation does not match the actual filename

Desired behavior after PR is merged

1. Clicking over lucky_numbers.cpp properly redirects to the implementation.
2. The listed filename matches the implementation filename.