[Doc] Fix incorrect ReferenceField syntax in 2 more places

Author: slax57

docs/ReferenceField.md

@@ -250,7 +250,7 @@ const PostList = () => (
         <Datagrid>
             <TextField source="title" />
             {/** renders without an additional request */}
-            <ReferenceField source="author_id" />
+            <ReferenceField source="author_id" reference="authors" />
         </Datagrid>
     </List>
 );

packages/ra-data-fakerest/README.md

@@ -185,7 +185,7 @@ const PostList = () => (
         <Datagrid>
             <TextField source="title" />
             {/** renders without an additional request */}
-            <ReferenceField source="author_id" />
+            <ReferenceField source="author_id" reference="authors" />
         </Datagrid>
     </List>
 );