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/README.md

@@ -23,8 +23,8 @@ The `addPlugin` accepts a `plugin` parameter, that registers a client plugin and
 
 ## Example
 
-An example plugin can be found in [FlipperTicTacToe.js](https://github.com/facebook/flipper/blob/master/react-native/ReactNativeFlipperExample/FlipperTicTacToe.js).
+An example plugin can be found in [FlipperTicTacToe.js](https://github.com/facebook/flipper/blob/main/react-native/ReactNativeFlipperExample/FlipperTicTacToe.js).
 
 The corresponding Desktop plugin ships by default in Flipper, so importing the above file and dropping the `<FlipperTicTacToe />` component somewhere in your application should work out of the box.
 
-The sources of the corresponding Desktop plugin can be found [here](https://github.com/facebook/flipper/tree/master/desktop/plugins/rn-tic-tac-toe).
+The sources of the corresponding Desktop plugin can be found [here](https://github.com/facebook/flipper/tree/main/desktop/plugins/rn-tic-tac-toe).

react-native/react-native-flipper/android/build.gradle

@@ -98,7 +98,7 @@ def configureReactNativePom(def pom) {
         licenses {
             license {
                 name packageJson.license
-                url packageJson.repository.baseUrl + '/blob/master/' + packageJson.licenseFilename
+                url packageJson.repository.baseUrl + '/blob/main/' + packageJson.licenseFilename
                 distribution 'repo'
             }
         }

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",