[Voter] Use the subject's FQCN in the support method

People could replace only the `BlogPost` class. So the support method will always return `false`.

With a leading `\`, the class name will resolve "normally"

Author: lyrixx

src/Resources/skeleton/security/Voter.tpl.php

@@ -13,7 +13,7 @@ protected function supports($attribute, $subject)
         // replace with your own logic
         // https://symfony.com/doc/current/security/voters.html
         return in_array($attribute, ['POST_EDIT', 'POST_VIEW'])
-            && $subject instanceof App\Entity\BlogPost;
+            && $subject instanceof \App\Entity\BlogPost;
     }
 
     protected function voteOnAttribute($attribute, $subject, TokenInterface $token)