docs(core): Fix typo in custom fields documentation (#2698)

docs/docs/guides/developer-guide/custom-fields/index.md

@@ -104,7 +104,7 @@ mutation {
 The custom fields will also extend the filter and sort options available to the `products` list query:
 
 ```graphql
-mutation {
+query {
     products(options: {
         // highlight-start
         filter: {