Fix docstring copy-paste error in maths/sumset.py

[satyamkumar-builds]

Describe your change:

The docstring of sumset() in maths/sumset.py contained a copy-paste error from maths/sylvester_sequence.py:

• The return description said "the nth number in Sylvester's sequence" — but the function computes a sumset, a completely different mathematical concept.
• The param names (first set, second set) did not match the actual parameter names (set_a, set_b).

Fixed both so the docstring now accurately describes the parameters and the return value:

:param set_a: a set of numbers
:param set_b: a set of numbers
:return: the sumset of set_a and set_b (all pairwise sums a + b)

No code logic changed — documentation fix only. All existing doctests pass.

Fixes #15013

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[priya-sundaram-dev]

LGTM — correct fix. The old :param first set: / :param second set: didn't match the real parameter names (set_a, set_b), and the :return: was copy-pasted from sylvester_sequence.py ("nth number in Sylvester's sequence") which has nothing to do with a sumset. The new text accurately describes the pairwise-sum set, and the existing doctests already cover behavior. Small, clean docs correction — thanks @satyamkumar-builds. 🤖 #ABotWroteThis

[cclauss]

Nice! Looks good to me.

[priya-sundaram-dev]

Approving. The old docstring was a copy-paste from a Sylvester's-sequence function and didn't match the code. The new :param set_a/:param set_b names line up with the actual signature, and the sumset of set_a and set_b (all pairwise sums a + b) correctly describes the behaviour — confirmed by the existing doctests ({1,2,3}+{4,5,6} → {5,6,7,8,9}). Docs-only, no behaviour change. LGTM. 👍