Fix typo in config docs for --extend-exclude (#3170)

The old regex in the example was invalid and caused an error on startup.

docs/usage_and_configuration/the_basics.md

@@ -265,7 +265,7 @@ extend-exclude = '''
 # in the root of the project.
 (
   ^/foo.py    # exclude a file named foo.py in the root of the project
-  | *_pb2.py  # exclude autogenerated Protocol Buffer files anywhere in the project
+  | .*_pb2.py  # exclude autogenerated Protocol Buffer files anywhere in the project
 )
 '''
 ```