minor #12379 Update routing.rst (AleksejJur)

This PR was squashed before being merged into the 4.3 branch (closes #12379).

Discussion
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Update routing.rst

Missing 'use Symfony\Component\Routing\Generator\UrlGenerator;' in Routing Component documentation

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Commits
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05493a7 Update routing.rst

Author: javiereguiluz

components/routing.rst

@@ -40,6 +40,7 @@ A routing system has three parts:
 Here is a quick example::
 
     use App\Controller\BlogController;
+    use Symfony\Component\Routing\Generator\UrlGenerator;
     use Symfony\Component\Routing\Matcher\UrlMatcher;
     use Symfony\Component\Routing\RequestContext;
     use Symfony\Component\Routing\Route;