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

desktop/pkg/schemas/plugin-package-v2.json

@@ -26,7 +26,7 @@
     },
     "icon": {
       "type": "string",
-      "description": "Determines the plugin icon which is displayed in the main sidebar. The list of available icons is static for now and can be found in https://github.com/facebook/flipper/blob/master/desktop/static/icons.json."
+      "description": "Determines the plugin icon which is displayed in the main sidebar. The list of available icons is static for now and can be found in https://github.com/facebook/flipper/blob/main/desktop/static/icons.json."
     },
     "keywords": {
       "description": "This helps people discover your package as it's listed in 'npm search'. To make the plugin discoverable in Flipper, the property must contain \"flipper-plugin\" keyword.",