[1.6.x] Fixed #22093 -- Made Form.Meta.fields examples consistent.

Thanks david-schultz at gmx.net for the suggestion.

Backport of 821fc925f0 from master

docs/topics/auth/customizing.txt

@@ -1078,7 +1078,7 @@ code would be required in the app's ``admin.py`` file::
 
         class Meta:
             model = MyUser
-            fields = ['email', 'password', 'date_of_birth', 'is_active', 'is_admin']
+            fields = ('email', 'password', 'date_of_birth', 'is_active', 'is_admin')
 
         def clean_password(self):
             # Regardless of what the user provides, return the initial value.