fix typo for build_conn/0 in testing controllers guide (#2667)
fix typo for build_conn/0 in testing controllers guide (#2667)

* Remove "m" typo from build_conn/0 command

* Start new sentence explaining pattern matching of conn from build_conn

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