Complement note about facades not to be mocked

Config cannot be mocked.

mocking.md

@@ -315,4 +315,4 @@ We can mock the call to the `Cache` facade by using the `shouldReceive` method,
         }
     }
 
-> {note} You should not mock the `Request` facade. Instead, pass the input you desire into the HTTP helper methods such as `call` and `post` when running your test.
+> {note} You should not mock the `Request` facade. Instead, pass the input you desire into the HTTP helper methods such as `call` and `post` when running your test. Also notice that you cannot mock the `Config` facade. Instead use `Config::set($key,$value)` in your test.