MacOS -> macOS

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.
- [X] Match the **code formatting** of the rest of the codebase.
- [X] Target the `master` branch, NOT a "stable" branch.

it has been just annoying me for a long time.
its `Mac` but its `macOS`.
https://www.apple.com/macos/sierra/

no testing needed

o be merged.

For more info, see the ["Pull Requests"][5] section of our "Contributing" guidelines.

[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
[4]: http://circleci.com/gh/facebook/react-native
[5]: https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests
Closes #13896

Differential Revision: D5036473

Pulled By: javache

fbshipit-source-id: e8f0813d67648da27af932a72e84e744c5ae84da

.github/ISSUE_TEMPLATE.md

@@ -32,5 +32,5 @@ What needs to be done to address this issue? Ideally, provide a pull request wit
 
 * React Native version: [FILL THIS OUT: Be specific, filling out "latest" here is not enough.]
 * Platform: [FILL THIS OUT: iOS, Android, or both?]
-* Development Operating System: [FILL THIS OUT: Are you developing on MacOS, Linux, or Windows?]
+* Development Operating System: [FILL THIS OUT: Are you developing on macOS, Linux, or Windows?]
 * Dev tools: [FILL THIS OUT: Xcode or Android Studio version, iOS or Android SDK version, if applicable]