docs/Formula-Cookbook: Fix outdated HEAD hash syntax
docs/Formula-Cookbook: Fix outdated HEAD hash syntax

- Using the previous syntax, `brew audit --strict` fails because this
  uses the old Ruby hash syntax.

EricFromCanada/brew,nandub/brew,Homebrew/brew,Homebrew/brew,claui/brew,JCount/brew,nandub/brew,JCount/brew,MikeMcQuaid/brew,claui/brew,nandub/brew,Homebrew/brew,JCount/brew,EricFromCanada/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,claui/brew,MikeMcQuaid/brew,Homebrew/brew,claui/brew,nandub/brew,JCount/brew,EricFromCanada/brew,EricFromCanada/brew