Fix Staging::WorkflowPolicy

Just because you are maintainer of a project, doesn't mean
you are able to create it. However if you can create it or
not doesn't matter in this case, only if you can change it.

src/api/app/policies/staging/workflow_policy.rb

@@ -6,7 +6,7 @@ def initialize(user, record)
   end
 
   def create?
-    ProjectPolicy.new(@user, @record.project).create?
+    ProjectPolicy.new(@user, @record.project).update?
   end
 
   def update?