Update HAXM installation link

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

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Closes https://github.com/facebook/react-native/pull/13434

Differential Revision: D4866808

fbshipit-source-id: 9259eff382d9440754e672ad05828342091877d2

docs/GettingStarted.md

@@ -196,7 +196,7 @@ Android Virtual Devices allow you to run Android apps on your computer without t
 
 Then, click "Next" to install all of these components.
 
-> If you've already installed Android Studio before, you can still [install HAXM](https://software.intel.com/en-us/android/articles/installation-instructions-for-intel-hardware-accelerated-execution-manager-windows) without performing a custom installation.
+> If you've already installed Android Studio before, you can still install HAXM ([Windows](https://software.intel.com/en-us/android/articles/installation-instructions-for-intel-hardware-accelerated-execution-manager-windows)|[Mac](https://software.intel.com/en-us/android/articles/installation-instructions-for-intel-hardware-accelerated-execution-manager-mac-os-x)) without performing a custom installation.
 
 <block class="linux android" />