Add missing prepositions to tutorial.md (#993)

Author: nht007

content/tutorial/tutorial.md

@@ -519,11 +519,11 @@ After these changes, we're again able to click on the Squares to fill them. Howe
 
 Since the Square components no longer maintain state, the Square components receive values from the Board component and inform the Board component when they're clicked. In React terms, the Square components are now **controlled components**. The Board has full control over them.
 
-Note how in `handleClick`, we call `.slice()` to create a copy the `squares` array to modify instead of modifying the existing array. We will explain why we create a copy of the `squares` array in the next section.
+Note how in `handleClick`, we call `.slice()` to create a copy of the `squares` array to modify instead of modifying the existing array. We will explain why we create a copy of the `squares` array in the next section.
 
 ### Why Immutability Is Important
 
-In the previous code example, we suggested that you use the `.slice()` operator to create a copy the `squares` array to modify instead of modifying the existing array. We'll now discuss immutability and why immutability is important to learn.
+In the previous code example, we suggested that you use the `.slice()` operator to create a copy of the `squares` array to modify instead of modifying the existing array. We'll now discuss immutability and why immutability is important to learn.
 
 There are generally two approaches to changing data. The first approach is to *mutate* the data by directly changing the data's values. The second approach is to replace the data with a new copy which has the desired changes.