Fix sumset docstring - #15038

Describe your change:

Fixes #15013

Corrected the sumset() docstring:

• Fixed the parameter names to set_a and set_b.
• Corrected the return-value description.
• Removed the incorrect reference to Sylvester's sequence.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests that will not be merged if they fail the automated tests.
• This PR only changes one algorithm file.
• 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.
• If this pull request resolves one or more open issues then the description above includes the issue number with a closing keyword: "Fixes #ISSUE-NUMBER".