Update doc/cookbook: use $HOMEBREW_PREFIX
Update doc/cookbook: use $HOMEBREW_PREFIX

Use $HOMEBREW_PREFIX instead of `/usr/local` when talking about
formula file path.

Closes Homebrew/homebrew#34568.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

pseudocody/brew,maxim-belkin/brew,alyssais/brew,pseudocody/brew,pseudocody/brew,JCount/brew,EricFromCanada/brew,hanxue/linuxbrew,maxim-belkin/brew,mistydemeo/homebrew,gregory-nisbet/brew,palxex/brew,Linuxbrew/brew,DomT4/brew,ilovezfs/brew,aw1621107/brew,sjackman/homebrew,toonetown/homebrew,maxim-belkin/brew,tonyg/homebrew,JCount/brew,jmsundar/brew,konqui/brew,AnastasiaSulyagina/brew,mistydemeo/homebrew,claui/brew,vitorgalvao/brew,bfontaine/brew,mistydemeo/homebrew,Linuxbrew/brew,tonyg/homebrew,konqui/brew,amar-laksh/brew_sudo,rwhogg/brew,Linuxbrew/brew,reelsense/brew,EricFromCanada/brew,claui/brew,claui/brew,mgrimes/brew,MikeMcQuaid/brew,gordonmcshane/homebrew,toonetown/homebrew,ilovezfs/brew,Homebrew/brew,mahori/brew,aw1621107/brew,nandub/brew,gregory-nisbet/brew,reitermarkus/brew,zmwangx/brew,zmwangx/brew,EricFromCanada/brew,palxex/brew,MikeMcQuaid/brew,sjackman/homebrew,hanxue/linuxbrew,nandub/brew,gordonmcshane/homebrew,muellermartin/dist,Linuxbrew/brew,reelsense/brew,alyssais/brew,muellermartin/dist,gordonmcshane/homebrew,sjackman/homebrew,vitorgalvao/brew,reitermarkus/brew,nandub/brew,reelsense/brew,zmwangx/brew,sjackman/homebrew,jmsundar/brew,EricFromCanada/brew,mgrimes/brew,DomT4/brew,amar-laksh/brew_sudo,reitermarkus/brew,vitorgalvao/brew,rwhogg/brew,JCount/brew,staticfloat/brew,alyssais/brew,tonyg/homebrew,JCount/brew,palxex/brew,rwhogg/brew,bfontaine/brew,mahori/brew,jmsundar/brew,ilovezfs/brew,AnastasiaSulyagina/brew,mgrimes/brew,aw1621107/brew,Homebrew/brew,Homebrew/brew,DomT4/brew,MikeMcQuaid/brew,gregory-nisbet/brew,staticfloat/brew,DomT4/brew,hanxue/linuxbrew,konqui/brew,amar-laksh/brew_sudo,staticfloat/brew,bfontaine/brew,AnastasiaSulyagina/brew,muellermartin/dist,toonetown/homebrew,mahori/brew,konqui/brew,Homebrew/brew,nandub/brew,vitorgalvao/brew,reitermarkus/brew,MikeMcQuaid/brew,mahori/brew,claui/brew