Change "header" to "heading" in hello-world.html

h1-h6 elements are called "headings", not "headers" (source: https://www.w3.org/TR/html52/sections.html#headings-and-sections).

content/docs/hello-world.md

@@ -23,7 +23,7 @@ ReactDOM.render(
 );
 ```
 
-It renders a header saying "Hello, world!" on the page.
+It renders a heading 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.