docs(auth): typo in authentication.rst (#19)

facebook login function name was "login_google" changed to "login_facebook

Author: nnicknamename

docs/guide/authentication.rst

@@ -229,7 +229,7 @@ with **Facebook** preset as the provider to use.
 
    # Example usage with Flask
    @auth.route('/login/facebook')
-   def login_google():
+   def login_facebook():
       return redirect(auth.authenticate_login_with_facebook())
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