small fix

book/http_cache.rst

@@ -396,7 +396,7 @@ The HTTP specification defines two caching models:
   until the cached version reaches its expiration time and becomes "stale".
 
 * When pages are really dynamic (i.e. their representation changes often),
-  the `validation model`_ model is often necessary. With this model, the
+  the `validation model`_ is often necessary. With this model, the
   cache stores the response, but asks the server on each request whether
   or not the cached response is still valid. The application uses a unique
   response identifier (the ``Etag`` header) and/or a timestamp (the ``Last-Modified``