fix typo

Author: yaacovCR

spec/Section 6 -- Execution.md

@@ -528,7 +528,7 @@ CollectFields(objectType, selectionSet, variableValues, visitedFragments):
       with the next {selection} in {selectionSet}.
     - Let {fragmentSelectionSet} be the top-level selection set of {fragment}.
     - Let {fragmentGroupedFieldSet} be the result of calling
-      {CollectFields(objectType, fragmentSelectionSet, variableValues,
+      {CollectFields(objectType, fieldSelectionSet, variableValues,
       visitedFragments)}.
     - For each {fragmentGroup} in {fragmentGroupedFieldSet}:
       - Let {responseKey} be the response key shared by all fields in