renamed gcd() to greatest_comman_divisor()

[ksharma20]

Fixed naming convention as described in Contribution.md

Expand acronyms because gcd() is hard to understand but greatest_common_divisor() is not.

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[dhruvmanila]

Thank you! 🎉

[ksharma20]

Thank you! 🎉

I am really glad @dhruvmanila, that I was able to contribute.
I am new to this open source contributing environment.
But, I still would like to learn more and contribute even further.
So can you guide me, what are other things or places where I can contribute or help ?
And Assign me some tasks etc If possible ?