fix typo in "Merging Selection Sets" section

Author: yaacovCR

spec/Section 6 -- Execution.md

@@ -774,8 +774,8 @@ sub-selections.
 ```
 
 After resolving the value for `me`, the selection sets are merged together by
-the by calling {CollectSubfields()} so `firstName` and `lastName` can be
-resolved for one value.
+calling {CollectSubfields()} so `firstName` and `lastName` can be resolved for
+one value.
 
 ### Handling Field Errors