Update usage.rst

`to_review` is a name of a transitions in this doc page.

But in the `workflow_has_marked_place` we pass a `place`, in this context it should be `review` instead of `to_review`

Author: Hakihiro

workflow/usage.rst

@@ -431,7 +431,7 @@ The following example shows these functions in action:
     {% endfor %}
 
     {# Check if the object is in some specific place #}
-    {% if workflow_has_marked_place(post, 'to_review') %}
+    {% if workflow_has_marked_place(post, 'review') %}
         <p>This post is ready for review.</p>
     {% endif %}