docs($http): fix link typo in $http doc

Should be $httpBackend instead of $httpBacked

Closes angular#1516

Author: jtymes

src/ng/http.js

@@ -172,7 +172,7 @@ function $HttpProvider() {
     /**
      * @ngdoc function
      * @name ng.$http
-     * @requires $httpBacked
+     * @requires $httpBackend
      * @requires $browser
      * @requires $cacheFactory
      * @requires $rootScope