Moved relu.py from maths/ to neural_network/activation_functions

[piyush-poddar]

Fixes #9048

Describe your change:

I have moved relu.py from maths/ to neural_network/activation_functions as suggested, and also updated DIRECTORY.md accordingly.

• 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 include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[tianyizheng02]

For clarify, could you also rename the file to rectified_linear_unit.py?

[piyush-poddar]

I have renamed file relu.py to rectified_linear_unit.py and also updated DIRECTORY.md accordingly.

Please review. Thanks!