docs(http): use http.request(FOO) instead of http(FOO)

Closes #4641

modules/angular2/src/http/backends/xhr_backend.ts

@@ -101,7 +101,7 @@ export class XHRConnection implements Connection {
  * })
  * class MyComponent {
  *   constructor(http:Http) {
- *     http('people.json').subscribe(res => this.people = res.json());
+ *     http.request('people.json').subscribe(res => this.people = res.json());
  *   }
  * }
  * ```