dispatch-build-bottle: use main branch

This is really only used in Homebrew/core, so it should be safe to just
change `ref` here to `main`. Without this, `dispatch-build-bottle`
creates PRs that mistakenly target the `master` branch instead of the
`main` branch.

Library/Homebrew/dev-cmd/dispatch-build-bottle.rb

@@ -42,7 +42,7 @@ module Homebrew
       def run
         tap = Tap.fetch(args.tap || CoreTap.instance.name)
         user, repo = tap.full_name.split("/")
-        ref = "master"
+        ref = "main"
         workflow = args.workflow || "dispatch-build-bottle.yml"
 
         runners = []