docs(guide/concepts): use protocol relative URL

When accessing the docs from https, the "Accessing the backend example fails
because it contains a hard coded protocol. By making the URL protocol relative,
the example should work over http and https.

docs/content/guide/concepts.ngdoc

@@ -323,7 +323,7 @@ The following example shows how this is done with Angular:
     angular.module('finance3', [])
       .factory('currencyConverter', ['$http', function($http) {
         var YAHOO_FINANCE_URL_PATTERN =
-              'http://query.yahooapis.com/v1/public/yql?q=select * from '+
+              '//query.yahooapis.com/v1/public/yql?q=select * from '+
               'yahoo.finance.xchange where pair in ("PAIRS")&format=json&'+
               'env=store://datatables.org/alltableswithkeys&callback=JSON_CALLBACK';
         var currencies = ['USD', 'EUR', 'CNY'];