Modified beta.openbadges issuer to backpack.openbadges

Hi Whitmer, 

I did just a tiny modification to use the "backpack.openbadges.org" issuer and no longer the "beta" version. 
It's the address that you can now see in the issuer API of backpack: (in the "using the API part")
https://github.com/mozilla/openbadges/wiki/Issuer-API

I hope it'll help! 

Kulgar.