contributing.md: Fix typo of flake8.

Author: schwehr

docs/contributing.md

@@ -78,7 +78,7 @@ Ready to contribute? Here's how to set up _geemap_ for local development.
 5. When you're done making changes, check that your changes pass flake8 and the tests, including testing other Python versions with tox:
 
     ```
-    pflake8 geemap tests
+    flake8 geemap tests
     ```
 
     ```