Update maximum_subarray.py

[pronoym99]

1. Rectify documentation to indicate the correct output: function doesn't return the subarray, but rather returns a sum.
2. Make the function more Pythonic and thereby avoid
   • any unnecessary if statements.
   • usage of the non - optimal in range(len()) constructs.
3. Make function annotation generic i.e. it can accept any iterable nums.
4. Raise value error when the input iterable nums is empty.

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}.