Merge pull request #3345 from xabbuh/fix-build-errors

[Book][Validation] fix code block syntax

Author: weaverryan

book/validation.rst

@@ -115,7 +115,7 @@ The job of the ``validator`` is easy: to read the constraints (i.e. rules)
 of a class and verify whether or not the data on the object satisfies those
 constraints. If validation fails, a non-empty list of errors
 (class :class:`Symfony\\Component\\Validator\\ConstraintViolationList`) is
-returned. Take this simple example from inside a controller:
+returned. Take this simple example from inside a controller::
 
     // ...
     use Symfony\Component\HttpFoundation\Response;