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)

Follow-up to pandas-devgh-15712

[ci skip]

doc/source/contributing.rst

@@ -527,7 +527,7 @@ unused function. However, style-checking the diff will not catch this because
 the actual import is not part of the diff. Thus, for completeness, you should
 run this command, though it will take longer::
 
-   git diff master --name-only -- '*.py' | grep 'pandas' | xargs -r flake8
+   git diff master --name-only -- '*.py' | grep 'pandas/' | xargs -r flake8
 
 Backwards Compatibility
 ~~~~~~~~~~~~~~~~~~~~~~~