bug #4165 Update voters.rst (gerryvdm)

This PR was submitted for the 2.5 branch but it was merged into the 2.4 branch instead (closes #4165).

Discussion
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Update voters.rst

Corrected the service name to be injected

Commits
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f0a46c9 Update voters.rst

cookbook/security/voters.rst

@@ -32,7 +32,7 @@ access, not to grant access.
 Creating a custom Voter
 -----------------------
 
-To blacklist a user based on its IP, you can use the ``request`` service
+To blacklist a user based on its IP, you can use the ``request_stack`` service
 and compare the IP address against a set of blacklisted IP addresses:
 
 .. code-block:: php