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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