Update configuration.rst

I found a mistake in this default configuration.
I have problem with the generation of thumbnail. When i try to replace 'liip_imagine.controller:filterAction' to liip_imagine.controller::filterAction that solve my problem.

So i think it's need to have '::' and not ':'.

Resources/doc/configuration.rst

@@ -40,8 +40,8 @@ The default configuration for the bundle looks like this:
             filters: []
             post_processors: []
         controller:
-            filter_action:          liip_imagine.controller:filterAction
-            filter_runtime_action:  liip_imagine.controller:filterRuntimeAction
+            filter_action:          liip_imagine.controller::filterAction
+            filter_runtime_action:  liip_imagine.controller::filterRuntimeAction
             redirect_response_code: 302
         webp:
             generate:    false