Updated hello world explanation message (#8703)

Output text in explanation now in sync with code sample

docs/hello-world.md

@@ -22,7 +22,7 @@ ReactDOM.render(
 );
 ```
 
-It renders a header saying "Hello World" on the page.
+It renders a header saying "Hello, world!" on the page.
 
 The next few sections will gradually introduce you to using React. We will examine the building blocks of React apps: elements and components. Once you master them, you can create complex apps from small reusable pieces.