DOC: Patch new flake8 command grep
The grep was initially matching to "pandas," which is incorrect because that was also matching files containing "pandas" in the name but that were not in the main `pandas` directory (e.g. performance test code).  This change enforces that we match to any Python files in the main `pandas` directory.

Also picked up compatibility issue with OSX, in which the `-r` flag does not exist.  However, `xargs` terminates if the argument list is empty, which was the whole point of passing in `-r` in the first place.

Follow-up to #15712