fix: correct sumset() docstring

[deepshekhardas]

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add, change, or clarify documentation?

What does this implement/fix?

The docstring of sumset() in maths/sumset.py was a copy-paste from sylvester_sequence.py. Correct the param names and return description to describe the sumset (all pairwise sums a + b).

Additional comments?

None.

Fixes #15013

[priya-sundaram-dev]

Good catch — the old docstring wrongly described Sylvester's sequence. This fix is clean and mergeable. Note there are two sibling PRs fixing the same docstring: #15014 is byte-identical to this one, and #15038 adds an extra empty-set doctest but is currently conflicting. Recommending this one as the keeper; the other two can be closed once this lands (worth cherry-picking #15038's sumset(set(), …) doctest here first).