bump-formula-pr: --message: add newlines before and after hr

Without an empty line before the horizontal rule (---), "Created with `brew
bump-formula-pr`." is treated as a setext header.

The newline before the hr is not part of the heredoc because putting it in
would trip up Rubocop.

Library/Homebrew/dev-cmd/bump-formula-pr.rb

@@ -335,8 +335,9 @@ module Homebrew
         EOS
         user_message = ARGV.value("message")
         if user_message
-          pr_message += <<~EOS
+          pr_message += "\n" + <<~EOS
             ---
+
             #{user_message}
           EOS
         end