fixed link to React Native Express (react-navigation#1208)

It seems to have moved from http://reactnativeexpress.com to https://www.reactnative.express

versioned_docs/version-6.x/getting-started.md

@@ -12,7 +12,7 @@ If you're already familiar with JavaScript, React and React Native, then you'll
 
 Here are some resources to help you out:
 
-1. [React Native Express](http://reactnativeexpress.com/) (Sections 1 to 4)
+1. [React Native Express](https://www.reactnative.express) (Sections 1 to 4)
 2. [Main Concepts of React](https://reactjs.org/docs/hello-world.html)
 3. [React Hooks](https://reactjs.org/docs/hooks-intro.html)
 4. [React Context](https://reactjs.org/docs/context.html) (Advanced)