Update plug name from find_message to fetch_message in plug guide
Update plug name from find_message to fetch_message in plug guide

`find_message/1` is used inside the plug which was previously called
`find_message/2` - this was potentially confusing.

eksperimental/phoenix,michalmuskala/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,pap/phoenix,0x00evil/phoenix,Kosmas/phoenix,evax/phoenix,mschae/phoenix,Gazler/phoenix,keathley/phoenix,nshafer/phoenix,jwarwick/phoenix,Joe-noh/phoenix,mitchellhenke/phoenix,nshafer/phoenix,optikfluffel/phoenix,stevedomin/phoenix,evax/phoenix,keathley/phoenix,rafbgarcia/phoenix,pap/phoenix,phoenixframework/phoenix,mitchellhenke/phoenix,eksperimental/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,0x00evil/phoenix,optikfluffel/phoenix,phoenixframework/phoenix,Gazler/phoenix,dmarkow/phoenix,nshafer/phoenix,gjaldon/phoenix,jwarwick/phoenix,stevedomin/phoenix,dmarkow/phoenix,mschae/phoenix,Joe-noh/phoenix,gjaldon/phoenix,michalmuskala/phoenix,mobileoverlord/phoenix,Kosmas/phoenix,mobileoverlord/phoenix