Point users to the npm page instead of the github project for prop-types (#9373)

docs/typechecking-with-proptypes.md

@@ -7,7 +7,7 @@ redirect_from:
 ---
 
 > Note:
-> `React.PropTypes` is deprecated as of React v15.5. Please use [the `prop-types` library instead](https://github.com/aackerman/PropTypes).
+> `React.PropTypes` is deprecated as of React v15.5. Please use [the `prop-types` library instead](https://www.npmjs.com/package/prop-types).
 
 As your app grows, you can catch a lot of bugs with typechecking. For some applications, you can use JavaScript extensions like [Flow](https://flowtype.org/) or [TypeScript](https://www.typescriptlang.org/) to typecheck your whole application. But even if you don't use those, React has some built-in typechecking abilities. To run typechecking on the props for a component, you can assign the special `propTypes` property: