[Form] Fix a minor syntax issue

Author: javiereguiluz

reference/forms/types/collection.rst

@@ -314,7 +314,7 @@ prototype_options
 
 This is the array that's passed to the form type specified in the `entry_type`_
 option when creating its prototype. It allows to have different options depending
-on whether you are adding a new entry or editing an existing entry.
+on whether you are adding a new entry or editing an existing entry::
 
     use Symfony\Component\Form\Extension\Core\Type\CollectionType;
     use Symfony\Component\Form\Extension\Core\Type\TextType;