[Editorial] Fix example for Variable Uniqueness (#703)

I think the fragment definition in this example was wrong

Author: PascalSenn

spec/Section 5 -- Validation.md

@@ -1552,7 +1552,7 @@ query B($atOtherHomes: Boolean) {
   ...HouseTrainedFragment
 }
 
-fragment HouseTrainedFragment {
+fragment HouseTrainedFragment on Query {
   dog {
     isHousetrained(atOtherHomes: $atOtherHomes)
   }