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/flipper-plugin/src/data-source/README.md

@@ -12,7 +12,7 @@ _Library to power streamig data visualisations as used in Facebook's Flipper_
 * [Demo sandbox](https://codesandbox.io/s/flipper-datasource-demo-iy0tq)
 * [Demo setup recording](https://www.youtube.com/watch?v=stL66GByQU0)
 * [In-depth introduction talk, featured at @fwdays, 5-6-2021](https://www.youtube.com/watch?v=WCs09c1B1Lk)
-* [Source code](https://github.com/facebook/flipper/tree/master/desktop/flipper-plugin/src/data-source)
+* [Source code](https://github.com/facebook/flipper/tree/main/desktop/flipper-plugin/src/data-source)
 * [Lightning talk using DataSource in Logs view](https://fb.workplace.com/groups/427492358561913/permalink/432720091372473/) [FB-Only]
 * [DataSource project plan](https://fb.quip.com/noJDArpLF7Fe) [FB-Only]
 * [Demo project](https://www.internalfb.com/code/flipper/src/fbsource/xplat/sonar/facebook/data-source-demo/) [FB-Only]

desktop/flipper-plugin/src/data-source/package.json

@@ -3,7 +3,7 @@
   "version": "0.0.2",
   "description": "Library to power streamig data visualisations",
   "repository": "https://github.com/facebook/flipper",
-  "homepage": "https://github.com/facebook/flipper/blob/master/desktop/flipper-plugin/src/data-source/README.md",
+  "homepage": "https://github.com/facebook/flipper/blob/main/desktop/flipper-plugin/src/data-source/README.md",
   "author": "Facebook Inc",
   "license": "MIT",
   "source": "index.tsx",