Updated sentence grammar. (#2459)

content/tutorial/tutorial.md

@@ -290,7 +290,7 @@ class Square extends React.Component {
 
 >Note
 >
->In [JavaScript classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes), you need to always call `super` when defining the constructor of a subclass. All React component classes that have a `constructor` should start it with a `super(props)` call.
+>In [JavaScript classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes), you need to always call `super` when defining the constructor of a subclass. All React component classes that have a `constructor` should start with a `super(props)` call.
 
 Now we'll change the Square's `render` method to display the current state's value when clicked: