Fix Cython formatting command in Contributing.md. (#8496)

This PR closes issue #8123. It removes a slight error in the Cython command in the CuDF `contributing.md` page.

Authors:
  - Marlene  (https://github.com/marlenezw)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)

URL: #8496

CONTRIBUTING.md

@@ -87,7 +87,7 @@ and then check the syntax of your Python and Cython code by running:
 
 ```bash
 flake8 python
-flake8 --config=python/cudf/.flake8.cython
+flake8 --config=python/.flake8.cython
 ```
 
 Additionally, many editors have plugins that will apply `isort` and `Black` as