Removed unneeded OS reference on running the test suite in contributing docs.

This is not needed as the console snippet has buttons that allows the user to choose their OS.

docs/intro/contributing.txt

@@ -217,8 +217,7 @@ a dependency for one or more of the Python packages. Consult the failing
 package's documentation or search the web with the error message that you
 encounter.
 
-Now we are ready to run the test suite. If you're using GNU/Linux, macOS, or
-some other flavor of Unix, run:
+Now we are ready to run the test suite:
 
 .. console::