README: fix missing guard around liquidprompt for bashrc instructions
README: fix missing guard around liquidprompt for bashrc instructions

Fixes https://github.com/nojhan/liquidprompt/issues/291

colinlieberman/liquidprompt,dpilone/liquidprompt,alexprengere/liquidprompt,dtschan/liquidprompt,tsbarnes/liquidprompt,joshuarubin/dotfiles,jonathanjsimon/liquidprompt,tstone2077/liquidprompt,mathieumd/liquidprompt,illwieckz/liquidprompt,milouse/liquidprompt,joshuarubin/dotfiles,tkuminecz/liquidprompt,lyoshenka/liquidprompt,joshuarubin/dotfiles,BobHood/liquidprompt,laurentbardin/liquidprompt,smadhueagle/liquidprompt,kenrachynski/liquidprompt,tocomo/liquidprompt,dan-cleinmark/liquidprompt,nojhan/liquidprompt,pkkolos/liquidprompt,joshuarubin/dotfiles,jaesivsm/liquidprompt,elementalvoid/liquidprompt,enrdstrm/liquidprompt,timhyson/liquidprompt