minor #15138 Typo with backticks. (askeita)

This PR was submitted for the 5.2 branch but it was merged into the 4.4 branch instead.

Discussion
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Typo with backticks.

Removed extra backticks that are visible.

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Commits
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e1ca152 Typo with backticks.

Author: OskarStark

http_cache/validation.rst

@@ -106,7 +106,7 @@ doing so much work.
 
 .. tip::
 
-    Symfony also supports weak ``ETag``s by passing ``true`` as the second
+    Symfony also supports weak ``ETag`` s by passing ``true`` as the second
     argument to the
     :method:`Symfony\\Component\\HttpFoundation\\Response::setEtag` method.