Fix prefix example code
Fix prefix example code

Believe this needs to be `this.urlPrefix` instead of the existing `this.prefix`.
unchartedcode/ember-cli-mirage,samselikoff/ember-cli-mirage,unchartedcode/ember-cli-mirage,unchartedcode/ember-cli-mirage,makepanic/ember-cli-mirage,samselikoff/ember-cli-mirage,samselikoff/ember-cli-mirage,makepanic/ember-cli-mirage,unchartedcode/ember-cli-mirage,samselikoff/ember-cli-mirage