minor #5714 Remove unnecessary word from http_cache.rst (zatikbalazs)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5714).

Discussion
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Remove unnecessary word from http_cache.rst

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3, 2.4, 2.5, 2.6, 2.7
| Fixed tickets | -

Commits
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e0b32e5 Remove unnecessary word from http_cache.rst

Author: xabbuh

book/http_cache.rst

@@ -882,8 +882,8 @@ that data from its cache.
 
     If you want to use cache invalidation, have a look at the
     `FOSHttpCacheBundle`_. This bundle provides services to help with various
-    cache invalidation concepts, and also documents the configuration for the
-    a couple of common caching proxies.
+    cache invalidation concepts, and also documents the configuration for a
+    couple of common caching proxies.
 
 If one content corresponds to one URL, the ``PURGE`` model works well.
 You send a request to the cache proxy with the HTTP method ``PURGE`` (using