minor #5715 Add missing code tag (zatikbalazs)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5715).

Discussion
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Add missing code tag

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Commits
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cb40b75 Add missing code tag

Author: xabbuh

book/http_cache.rst

@@ -230,7 +230,7 @@ Here is a list of the main options:
 ``allow_revalidate``
     Specifies whether the client can force a cache revalidate by including a
     ``Cache-Control`` "max-age=0" directive in the request. Set it to ``true`` for
-    compliance with RFC 2616 (default: false).
+    compliance with RFC 2616 (default: ``false``).
 
 ``stale_while_revalidate``
     Specifies the default number of seconds (the granularity is the second as the