Correct warning

Currently, if you open up a action generated by the tap-new command in
the github GUI, you get a warning saying "pull_requests should be an
object". This fixes that.

This wasn't stopping anything working currently, though I suppose it may
in the future.

Co-authored-by: Issy Long <me@issyl0.co.uk>

Library/Homebrew/dev-cmd/tap-new.rb

@@ -50,7 +50,7 @@ module Homebrew
       on:
         push:
           branches: master
-        pull_request: []
+        pull_request:
       jobs:
         test-bot:
           runs-on: ${{ matrix.os }}