Update part8d.md

Grammar update

Author: myverdict

src/content/8/en/part8d.md

@@ -326,7 +326,7 @@ Implement a view which shows all the books based on the logged-in user's favouri
 #### 8.21 books by genre with GraphQL
 
 In the previous two exercises, the filtering could have been done using just React.
-To complete this exercise, you should redo the filtering the books based on a selected genre (that was done in exercise 8.19) using a GraphQL query to the server. If you already did so then you do not have to do anything.
+To complete this exercise, you should redo the filtering of the books based on a selected genre (that was done in exercise 8.19) using a GraphQL query to the server. If you already did so then you do not have to do anything.
 
 This and the next exercise are quite **challenging**, like they should be this late in the course. It may help you to complete the easier exercises in the [next part](/en/part8/fragments_and_subscriptions) before doing 8.21 and 8.22.