Update custom_authentication_provider.rst

Missing word on the introduction, I propose "manner", "way" will work too.

Author: Jérémy Crapet

security/custom_authentication_provider.rst

@@ -8,7 +8,7 @@ How to Create a custom Authentication Provider
 
     Creating a custom authentication system is hard, and this entry will walk
     you through that process. But depending on your needs, you may be able
-    to solve your problem in a simpler, or via a community bundle:
+    to solve your problem in a simpler manner, or via a community bundle:
 
     * :doc:`/security/custom_password_authenticator`
     * :doc:`/security/api_key_authentication`