fix(type): fix on both class and Doctrine type

Author: Guikingone

workflow/usage.rst

@@ -169,7 +169,7 @@ As configured, the following property is used by the marking store::
     If a ``multiple_state`` is used, the persisted state type should be an ``array``,
     using Doctrine, you can use ``json`` or ``json_array``.
 
-    If a ``single_state`` is used, the persisted state type could be a string, 
+    If a ``single_state`` is used, the persisted state type should be a string,
     if you need to store an int (due to DB limitations and/or project constraints),
     be aware that you need to implement :class:`Symfony\\Component\\Workflow\\MarkingStore\\MarkingStoreInterface`
     in order to transform the int back to something understandable by Workflow.