docs: replace command with function

Because it is a PHP user function.

Author: kenjis

docs/authentication.md

@@ -50,7 +50,7 @@ public $defaultAuthenticator = 'session';
 ## Auth Helper
 
 The auth functionality is designed to be used with the `auth_helper` that comes with Shield. This
-helper method provides the `auth()` command which returns a convenient interface to the most frequently
+helper method provides the `auth()` function which returns a convenient interface to the most frequently
 used functionality within the auth libraries.
 
 ```php