Fix typo (missing plural)

docs/advanced_topics/customisation/custom_user_models.rst

@@ -45,7 +45,7 @@ Create your custom user create and edit forms in your app:
       status = forms.ModelChoiceField(queryset=MembershipStatus.objects, required=True, label=_("Status"))
 
 
-Extend the Wagtail user create and edit templates. These extended template should be placed in a
+Extend the Wagtail user create and edit templates. These extended templates should be placed in a
 template directory ``wagtailusers/users``.
 
 Template create.html: