Fixed missing newline (W292)

wagtail/wagtailcore/compat.py

@@ -9,4 +9,4 @@ try:
     AUTH_USER_APP_LABEL, AUTH_USER_MODEL_NAME = AUTH_USER_MODEL.rsplit('.', 1)
 except ValueError:
     raise ImproperlyConfigured("AUTH_USER_MODEL must be of the form"
-                               " 'app_label.model_name'")
+                               " 'app_label.model_name'")