Rename default branch references to main (#2689)

Summary:
I really hope I hit everything and didn't break anything that wasn't
referring to branch names.

Pull Request resolved: https://github.com/facebook/flipper/pull/2689

Test Plan:
_eyes

CI

|[Site Preview: flipper](https://our.intern.facebook.com/intern/staticdocs/eph/D30305789/V2/flipper/)

Reviewed By: timur-valiev

Differential Revision: D30305789

Pulled By: passy

fbshipit-source-id: 5daaa09250bb96bb50be679fc01dae86c666eb73

react-native/react-native-flipper/package.json

@@ -11,7 +11,7 @@
   "repository": {
     "type": "git",
     "url": "git+https://github.com/facebook/flipper.git",
-    "baseUrl": "https://github.com/facebook/flipper/tree/master/react-native/react-native-flipper"
+    "baseUrl": "https://github.com/facebook/flipper/tree/main/react-native/react-native-flipper"
   },
   "keywords": [
     "react-native",