Update HAXM installation link

Summary:
Thanks for submitting a PR! Please read these instructions carefully:

- [x] Explain the **motivation** for making this change.
- [x] Provide a **test plan** demonstrating that the code is solid.
- [ ] Match the **code formatting** of the rest of the codebase.
- [x] Target the `master` branch, NOT a "stable" branch.

So other Mac users wont have to dig around to look for the link.

What existing problem does the pull request solve?

A good test plan has the exact commands you ran and their output, provides screenshots or videos if the pull request changes UI or updates the website. See [What is a Test Plan?][1] to learn more.

If you have added code that should be tested, add tests.

Sign the [CLA][2], if you haven't already.

Small pull requests are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

Make sure all **tests pass** on both [Travis][3] and
Closes https://github.com/facebook/react-native/pull/13434

Differential Revision: D4866808

fbshipit-source-id: 9259eff382d9440754e672ad05828342091877d2