[clang-tidy] Fix minor bug in add_new_check.py

While rebuilding the list of checks in add_new_check.py,
check is a file is a subdirectory before traversing it.

Test plan: Ran `./add_new_check.py --update-docs` and confirmed the list.rst file was unchanged.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D140772