Update task.md

Add semi colon for being more clear.

Author: QiyanYu

1-js/05-data-types/04-array/3-call-array-this/task.md

@@ -11,7 +11,7 @@ let arr = ["a", "b"];
 
 arr.push(function() {
   alert( this );
-})
+});
 
 arr[2](); // ?
 ```