s/Mock DOM/Virtual DOM/

Let's be consistent with the naming

docs/07-working-with-the-browser.md

@@ -10,7 +10,7 @@ next: more-about-refs.html
 React provides powerful abstractions that free you from touching the DOM directly in most cases, but sometimes you simply need to access the underlying API, perhaps to work with a third-party library or existing code.
 
 
-## The Mock DOM
+## The Virtual DOM
 
 React is so fast because it never talks to the DOM directly. React maintains a fast in-memory representation of the DOM. `render()` methods return a *description* of the DOM, and React can diff this description with the in-memory representation to compute the fastest way to update the browser.