minor symfony#9081 Use escaped backslashes (adursun)

This PR was submitted for the 4.0 branch but it was merged into the 2.7 branch instead (closes symfony#9081).

Discussion
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Use escaped backslashes

Commits
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ba4b720 Use escaped backslashes

Author: javiereguiluz

routing.rst

@@ -689,7 +689,7 @@ Troubleshooting
 
 Here are some common errors you might see while working with routing:
 
-    Controller "AppBundle\Controller\BlogController::showAction()" requires that you
+    Controller "AppBundle\\Controller\\BlogController::showAction()" requires that you
     provide a value for the "$slug" argument.
 
 This happens when your controller method has an argument (e.g. ``$slug``)::