Fix typo in CollectFields (#732)

Author: benjie

spec/Section 6 -- Execution.md

@@ -479,7 +479,7 @@ response in a stable and predictable order.
 
 CollectFields(objectType, selectionSet, variableValues, visitedFragments):
 
-  * If {visitedFragments} if not provided, initialize it to the empty set.
+  * If {visitedFragments} is not provided, initialize it to the empty set.
   * Initialize {groupedFields} to an empty ordered map of lists.
   * For each {selection} in {selectionSet}:
     * If {selection} provides the directive `@skip`, let {skipDirective} be that directive.