Typo with backticks.

Removed extra backticks.

Author: askeita

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.