[Book] [Security] Fix typo

Author: michaelperrin

book/security.rst

@@ -798,7 +798,7 @@ Take the following ``access_control`` entries as an example:
                 array('path' => '^/admin', 'role' => 'ROLE_USER'),
             ),
 
-For each incoming request, Symfony will decided which ``access_control``
+For each incoming request, Symfony will decide which ``access_control``
 to use based on the URI, the client's IP address, the incoming host name,
 and the request method. Remember, the first rule that matches is used, and
 if ``ip``, ``host`` or ``method`` are not specified for an entry, that ``access_control``