Updated Mutations section of Learn Queries (graphql#109)

Updated text to reflect changes in graphql mutation schema: removal of `incrementCredits`, addition of `createReview`.

Author: vahe

site/learn/Learn-Queries.md

@@ -218,7 +218,7 @@ mutation CreateReviewForEpisode($ep: Episode!, $review: ReviewInput!) {
 }
 ```
 
-In this case, the `incrementCredits` mutation field returns a `Human` object, so we can query the new value of `totalCredits` after giving that character some more credits. Otherwise, we would have needed to send two requests - one to update the credits, and another to get the new value - or guess at the new amount based on outdated data.
+Note how `createReview` field returns the `stars` and `commentary` fields of the newly created review. This is especially useful when mutating existing data, for example, when incrementing a field, since we can mutate and query the new value of the field with one request.
 
 You might also notice that, in this example, the `review` variable we passed in is not a scalar. It's an _input object type_, a special kind of object type that can be passed in as an argument. Learn more about input types on the Schema page.