[FrameworkBundle] fixes grammar in container:debug command manual.

Author: Hugo Hamon

Command/ContainerDebugCommand.php

@@ -75,7 +75,7 @@ protected function configure()
 
   <info>php %command.full_name% --parameters</info>
 
-Display a specific parameter by specifying his name with the <info>--parameter</info> option:
+Display a specific parameter by specifying its name with the <info>--parameter</info> option:
 
   <info>php %command.full_name% --parameter=kernel.debug</info>
 EOF