[Cache] Fix typo

Author: bicpi

book/http_cache.rst

@@ -535,7 +535,7 @@ example).
     The 304 status code means "Not Modified". It's important because with
     this status code the response does *not* contain the actual content being
     requested. Instead, the response is simply a light-weight set of directions that
-    tell the cache that it should use its stored version.
+    tells the cache that it should use its stored version.
 
 Like with expiration, there are two different HTTP headers that can be used
 to implement the validation model: ``ETag`` and ``Last-Modified``.