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

MikeMcQuaid/brew,palxex/brew,reitermarkus/brew,sjackman/homebrew,MikeMcQuaid/brew,mahori/brew,vitorgalvao/brew,maxim-belkin/brew,DomT4/brew,EricFromCanada/brew,jmsundar/brew,claui/brew,palxex/brew,pseudocody/brew,sjackman/homebrew,reelsense/brew,mgrimes/brew,MikeMcQuaid/brew,ilovezfs/brew,nandub/brew,JCount/brew,vitorgalvao/brew,alyssais/brew,nandub/brew,aw1621107/brew,reitermarkus/brew,claui/brew,reelsense/brew,EricFromCanada/brew,jmsundar/brew,ilovezfs/brew,jmsundar/brew,palxex/brew,Linuxbrew/brew,pseudocody/brew,alyssais/brew,JCount/brew,Linuxbrew/brew,konqui/brew,vitorgalvao/brew,DomT4/brew,maxim-belkin/brew,EricFromCanada/brew,Homebrew/brew,reelsense/brew,Linuxbrew/brew,konqui/brew,aw1621107/brew,JCount/brew,Linuxbrew/brew,mgrimes/brew,mahori/brew,konqui/brew,konqui/brew,hanxue/linuxbrew,ilovezfs/brew,JCount/brew,sjackman/homebrew,alyssais/brew,EricFromCanada/brew,aw1621107/brew,hanxue/linuxbrew,nandub/brew,Homebrew/brew,hanxue/linuxbrew,claui/brew,Homebrew/brew,vitorgalvao/brew,MikeMcQuaid/brew,sjackman/homebrew,mahori/brew,mgrimes/brew,Homebrew/brew,DomT4/brew,DomT4/brew,reitermarkus/brew,maxim-belkin/brew,nandub/brew,pseudocody/brew,claui/brew,mahori/brew,reitermarkus/brew