[libc] Fix typo of namespace in round implementation.

Summary: Fixed typos in namespace declarations and comments in the files under `src/math/round`.

Reviewers: sivachandra, abrachet

Reviewed By: sivachandra, abrachet

Subscribers: MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D75205