frontend cache purge docs change

docs/frontend_cache_purging.rst

@@ -107,9 +107,11 @@ Purging individual URLs
 
 ``wagtail.contrib.wagtailfrontendcache.utils`` provides another utils function called ``purge_url_from_cache``. As the name suggests, this purges an individual URL from the cache.
 
+For example, this could be useful for purging a single page of blogs:
+
 .. code-block:: python
 
     from wagtail.contrib.wagtailfrontendcache.utils import purge_url_from_cache
 
-    # Purge the homepage
-    purge_url_from_cache(homepage.full_url)
+    # Purge the first page of the blog index
+    purge_url_from_cache(blog_index.url + '?page=1')