Update learning.md (#3247)
Update learning.md (#3247)

Update link to point to new book (in beta).
phoenixframework/phoenix,keathley/phoenix,rafbgarcia/phoenix,mitchellhenke/phoenix,jwarwick/phoenix,rafbgarcia/phoenix,Joe-noh/phoenix,nshafer/phoenix,Gazler/phoenix,dmarkow/phoenix,0x00evil/phoenix,michalmuskala/phoenix,keathley/phoenix,optikfluffel/phoenix,michalmuskala/phoenix,optikfluffel/phoenix,eksperimental/phoenix,evax/phoenix,eksperimental/phoenix,nshafer/phoenix,0x00evil/phoenix,jwarwick/phoenix,evax/phoenix,mitchellhenke/phoenix,Gazler/phoenix,dmarkow/phoenix,nshafer/phoenix,phoenixframework/phoenix,phoenixframework/phoenix,Joe-noh/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,optikfluffel/phoenix