docs($provide): return instance of Ping, rather than constructor

Updating $provide.service method docs

The previous example provided for the service method did not work.  I've updated the example to a working example.

I think this version of the example will probably make more sense to most people, and the factory method would be
a better place for this sort of example.

Closes #6008

Author: clark-pan

src/auto/injector.js

@@ -482,17 +482,16 @@ function annotate(fn) {
  * Here is an example of registering a service using
  * {@link AUTO.$provide#methods_service $provide.service(class)}.
  * <pre>
- *   $provide.service('ping', ['$http', function($http) {
- *     var Ping = function() {
- *       this.$http = $http;
- *     };
- *   
- *     Ping.prototype.send = function() {
- *       return this.$http.get('/ping');
- *     }; 
+ *   var Ping = function($http) {
+ *     this.$http = $http;
+ *   };
+ * 
+ *   Ping.$inject = ['$http'];
  *   
- *     return Ping;
- *   }]);
+ *   Ping.prototype.send = function() {
+ *     return this.$http.get('/ping');
+ *   };
+ *   $provide.service('ping', Ping);
  * </pre>
  * You would then inject and use this service like this:
  * <pre>