[clang-tidy] correct clang-tidy-diff.py help message

Summary:
Looks like the original code was copied from clang-format-diff.py.
Update help message to make it clang-tidy specific.

Reviewers: klimek, alexfh

Subscribers: Eugene.Zelenko, cfe-commits

Patch by Igor Sugak!

Differential Revision: http://reviews.llvm.org/D21050

llvm-svn: 272144