release-notes: use stderr for header output.
release-notes: use stderr for header output.

This makes the use of `brew release-notes | pbcopy` a bit easier.

reitermarkus/brew,sjackman/homebrew,EricFromCanada/brew,Homebrew/brew,Homebrew/brew,EricFromCanada/brew,sjackman/homebrew,konqui/brew,vitorgalvao/brew,vitorgalvao/brew,konqui/brew,JCount/brew,MikeMcQuaid/brew,mahori/brew,vitorgalvao/brew,EricFromCanada/brew,vitorgalvao/brew,konqui/brew,claui/brew,MikeMcQuaid/brew,nandub/brew,konqui/brew,Homebrew/brew,mahori/brew,claui/brew,nandub/brew,reitermarkus/brew,Homebrew/brew,mahori/brew,MikeMcQuaid/brew,JCount/brew,nandub/brew,sjackman/homebrew,mahori/brew,claui/brew,claui/brew,MikeMcQuaid/brew,JCount/brew,nandub/brew,EricFromCanada/brew,sjackman/homebrew,JCount/brew,reitermarkus/brew,reitermarkus/brew