[Doc] Fix incorrect ReferenceField syntax

Author: fzaninotto

docs/DataProviders.md

@@ -418,7 +418,7 @@ const PostList = () => (
         <Datagrid>
             <TextField source="title" />
             {/** renders without an additional request */}
-            <ReferenceField source="author_id" />
+            <ReferenceField source="author_id" reference="authors" />
         </Datagrid>
     </List>
 );