Clarify mutability of sets in dictionary key explanation (Issue #192)

[anuragrawattt]

This pull request addresses Issue #192 by updating the documentation to clarify that sets, like lists and dictionaries, are mutable and therefore cannot be used as dictionary keys.

The original sentence omitted "sets" in the second clause. The revised version now reads:

"Neither a list, a set, nor another dictionary can serve as a dictionary key, because lists, sets, and dictionaries are mutable."

This improves clarity for learners and aligns with Python’s behavior regarding hashable types.

Fixes #192