Update heroku.md for Distributed tasks link (#3562)
Update heroku.md for Distributed tasks link (#3562)

Link for Distributed tasks was returning a 404. Updated to what I believe is the intended target.
nshafer/phoenix,keathley/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,0x00evil/phoenix,jwarwick/phoenix,phoenixframework/phoenix,keathley/phoenix,optikfluffel/phoenix,michalmuskala/phoenix,eksperimental/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,nshafer/phoenix,optikfluffel/phoenix,nshafer/phoenix,rafbgarcia/phoenix,jwarwick/phoenix,michalmuskala/phoenix,0x00evil/phoenix,phoenixframework/phoenix,0x00evil/phoenix,eksperimental/phoenix