executeFields: update grouped field set variable name (graphql#3896)

missed by me both in graphql#3891 and graphql#3878

Author: yaacovCR

src/execution/execute.ts

@@ -444,13 +444,13 @@ function executeFields(
   parentType: GraphQLObjectType,
   sourceValue: unknown,
   path: Path | undefined,
-  fields: GroupedFieldSet,
+  groupedFieldSet: GroupedFieldSet,
 ): PromiseOrValue<ObjMap<unknown>> {
   const results = Object.create(null);
   let containsPromise = false;
 
   try {
-    for (const [responseName, fieldGroup] of fields) {
+    for (const [responseName, fieldGroup] of groupedFieldSet) {
       const fieldPath = addPath(path, responseName, parentType.name);
       const result = executeField(
         exeContext,