Update part6d.md

Author: hanasiu

src/content/6/en/part6d.md

@@ -770,7 +770,7 @@ The notification is displayed for five seconds.
 
 #### Exercise 6.24.
 
-As stated in exercise 6.20, the server requires that the content of the anecdote to be added is at least 5 characters long. Now implement error handling for the insertion. In practice, it is sufficient to display a notification to the user in case of a failed POST request:
+As stated in exercise 6.21, the server requires that the content of the anecdote to be added is at least 5 characters long. Now implement error handling for the insertion. In practice, it is sufficient to display a notification to the user in case of a failed POST request:
 
 ![browser showing error notification for trying to add too short of an anecdoate](../../images/6/67new.png)