fix: signal receivers example raising exception

Signal receivers must accept `**kwargs`, otherwise one runs into this:

```
File "/lib/python3.8/site-packages/django/dispatch/dispatcher.py", line 94, in connect
raise ValueError("Signal receivers must accept keyword arguments (**kwargs).")
ValueError: Signal receivers must accept keyword arguments (**kwargs).
```

docs/reference/contrib/frontendcache.rst

@@ -192,12 +192,12 @@ This signal handler would trigger the invalidation of the index page using the
 
 
     @receiver(page_published, sender=BlogPage)
-    def blog_published_handler(instance):
+    def blog_published_handler(instance, **kwargs):
         blog_page_changed(instance)
 
 
     @receiver(pre_delete, sender=BlogPage)
-    def blog_deleted_handler(instance):
+    def blog_deleted_handler(instance, **kwargs):
         blog_page_changed(instance)