Fix the brunch configuration documentation link
Fix the brunch configuration documentation link

It was pointing to `stable` and 404'ing on GitHub. Switching it to
master fixes it.

dmarkow/phoenix,Gazler/phoenix,vic/phoenix,rafbgarcia/phoenix,mitchellhenke/phoenix,iStefo/phoenix,pap/phoenix,Joe-noh/phoenix,eksperimental/phoenix,korobkov/phoenix,phoenixframework/phoenix,michalmuskala/phoenix,jamilabreu/phoenix,vic/phoenix,phoenixframework/phoenix,Ben-Evolently/phoenix,CultivateHQ/phoenix,rafbgarcia/phoenix,mitchellhenke/phoenix,korobkov/phoenix,optikfluffel/phoenix,cas27/phoenix,CultivateHQ/phoenix,eksperimental/phoenix,michalmuskala/phoenix,keathley/phoenix,mschae/phoenix,cas27/phoenix,dmarkow/phoenix,optikfluffel/phoenix,iStefo/phoenix,ericmj/phoenix,gjaldon/phoenix,mschae/phoenix,evax/phoenix,jwarwick/phoenix,nshafer/phoenix,bsmr-erlang/phoenix,Kosmas/phoenix,phoenixframework/phoenix,Kosmas/phoenix,trarbr/phoenix,wsmoak/phoenix,0x00evil/phoenix,jwarwick/phoenix,nshafer/phoenix,ericmj/phoenix,stevedomin/phoenix,gjaldon/phoenix,pap/phoenix,ybur-yug/phoenix,Gazler/phoenix,0x00evil/phoenix,mobileoverlord/phoenix,ybur-yug/phoenix,Joe-noh/phoenix,rafbgarcia/phoenix,trarbr/phoenix,wsmoak/phoenix,0x00evil/phoenix,ephe-meral/phoenix,mobileoverlord/phoenix,evax/phoenix,optikfluffel/phoenix,stevedomin/phoenix,nshafer/phoenix,ephe-meral/phoenix,keathley/phoenix,jamilabreu/phoenix,bsmr-erlang/phoenix