fix missing argument to collect fields

Author: yaacovCR

spec/Section 6 -- Execution.md

@@ -563,7 +563,8 @@ Root field collection processes the operation's top-level selection set:
 CollectRootFields(rootType, operationSelectionSet, variableValues):
 
 - Initialize {groupedFieldSet} to an empty ordered map of lists.
-- Call {CollectFields(rootType, operationSelectionSet, groupedFieldSet)}.
+- Call {CollectFields(rootType, operationSelectionSet, variableValues
+  groupedFieldSet)}.
 - Return {groupedFieldSet}.
 
 ### Object Subfield Collection