minor #18707 [Cache] Add missing backtick (rylixs)

This PR was submitted for the 6.2 branch but it was merged into the 6.3 branch instead.

Discussion
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[Cache] Add missing backtick

👋

A backtick was missing on this part since version 6.2 of this page.

Commits
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0f1f5db [Cache] Add missing backtick

Author: javiereguiluz

http_cache/expiration.rst

@@ -92,7 +92,7 @@ The resulting HTTP header will look like this:
 
 .. note::
 
-    The ``expires` option and the ``setExpires()`` method automatically convert
+    The ``expires`` option and the ``setExpires()`` method automatically convert
     the date to the GMT timezone as required by the specification.
 
 Note that in HTTP versions before 1.1 the origin server wasn't required to