minor #1455 Syntax error in documentaiton Security Guard (pierreboissinot)

This PR was squashed before being merged into the 2.0.x-dev branch (closes #1455).

Discussion
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Syntax error in documentaiton Security Guard

"=>" replaced by "="

Commits
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71cee16a Syntax error in documentaiton Security Guard

doc/cookbook/guard_authentication.rst

@@ -136,7 +136,7 @@ Finally, configure your `security.firewalls` key to use this authenticator:
 
 .. code-block:: php
 
-    $app['security.firewalls'] => array(
+    $app['security.firewalls'] = array(
         'main' => array(
             'guard' => array(
                 'authenticators' => array(