Set source_url_pattern for installer
Set source_url_pattern for installer

This ensures that the links to the source in the documentation goes to

https://github.com/phoenixframework/phoenix/installer

Instead of:

https://github.com/phoenixframework/phoenix

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