docs: fix link in README (#2835)

Updated the link for the Getting Started Guide for React Native for
macOS.

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## Summary:

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problem does the pull request solve? -->
I noticed a link in the docs that was lead-piping users straight into a
dead end. I've updated it to point to the actual content, saving future
developers roughly **2.5 seconds of confusion** and one frustrated sigh
each.

Author: christophby

README.md

@@ -69,7 +69,7 @@ For a full and detailed list of the system requirements and how to set up your d
 
 ## Getting Started
 
-See the [Getting Started Guide](https://microsoft.github.io/react-native-windows/docs/rnm-getting-started) on our React Native for Windows + macOS website to build your first React Native for macOS app.
+See the [Getting Started Guide](https://microsoft.github.io/react-native-macos/docs/getting-started) on our React Native for macOS website to build your first React Native for macOS app.
 
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