Update docs to new method of adding a MIME (#1865)
Update docs to new method of adding a MIME (#1865)

This is how it is described in the Plug documentation because the old way is
no longer supported.
dmarkow/phoenix,jwarwick/phoenix,phoenixframework/phoenix,Gazler/phoenix,CultivateHQ/phoenix,michalmuskala/phoenix,trarbr/phoenix,jamilabreu/phoenix,evax/phoenix,ericmj/phoenix,iStefo/phoenix,optikfluffel/phoenix,nshafer/phoenix,wsmoak/phoenix,Gazler/phoenix,evax/phoenix,wsmoak/phoenix,Joe-noh/phoenix,mobileoverlord/phoenix,stevedomin/phoenix,mitchellhenke/phoenix,gjaldon/phoenix,nshafer/phoenix,mschae/phoenix,nshafer/phoenix,mobileoverlord/phoenix,Kosmas/phoenix,phoenixframework/phoenix,stevedomin/phoenix,iStefo/phoenix,jwarwick/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,rafbgarcia/phoenix,rafbgarcia/phoenix,pap/phoenix,optikfluffel/phoenix,CultivateHQ/phoenix,0x00evil/phoenix,bsmr-erlang/phoenix,cas27/phoenix,Kosmas/phoenix,phoenixframework/phoenix,pap/phoenix,keathley/phoenix,eksperimental/phoenix,cas27/phoenix,michalmuskala/phoenix,0x00evil/phoenix,Ben-Evolently/phoenix,jamilabreu/phoenix,dmarkow/phoenix,korobkov/phoenix,optikfluffel/phoenix,mschae/phoenix,Joe-noh/phoenix,gjaldon/phoenix,trarbr/phoenix,ybur-yug/phoenix,bsmr-erlang/phoenix,ericmj/phoenix,eksperimental/phoenix,mitchellhenke/phoenix,keathley/phoenix,korobkov/phoenix,ybur-yug/phoenix