Fixed v3 API config's billingUrl fetching
Fixed v3 API config's billingUrl fetching

refs https://github.com/TryGhost/Ghost/commit/26f56626ce93e2e1e718b3be68178d3eda217b11

- The renamed was missed during the config rename host_settings -> hostSettings in the referenced commit.

TryGhost/Ghost,janvt/Ghost,e10/Ghost,janvt/Ghost,ErisDS/Ghost,bitjson/Ghost,TryGhost/Ghost,ErisDS/Ghost,e10/Ghost,TryGhost/Ghost,bitjson/Ghost,ErisDS/Ghost