How to call service from different file in Angularjs

I know almost same kind of question is answered in couple of other posts but I'm getting "Unknown provider" error on calling service written in different file then callee Controller function. Could some one please help me to know what am I doing wrong? Here is my code

Controller.js

var mainApp = angular.module("mainApp", []);

mainApp.controller("PromoList", ['$scope', '$http', 'modelSerivce', function ($scope, $http, modelSerivce){
var serviceURL = modelSerivce.serviceURL();
console.log("from promolist controller: "+ serviceURL);}]);

Service.js

angular.module('mainApp').service('modelSerivce', function(){       
this.serviceURL = function()
{
    var serviceURL = “cofig.xml”;
    return serviceURL;
};});