docs(Http): add type annotations to clarify API

IMHO this tiny example is easier to read when some type annotations are added and the parameter names are more concise.

Closes #4614

modules/playground/src/http/http_comp.ts

@@ -1,5 +1,5 @@
 import {Component, View, NgFor} from 'angular2/angular2';
-import {Http} from 'angular2/http';
+import {Http, Response} from 'angular2/http';
 
 @Component({selector: 'http-app'})
 @View({
@@ -14,8 +14,10 @@ import {Http} from 'angular2/http';
   `
 })
 export class HttpCmp {
-  people: Object;
+  people: Object[];
   constructor(http: Http) {
-    http.get('./people.json').map(res => res.json()).subscribe(res => this.people = res);
+    http.get('./people.json')
+        .map((res: Response) => res.json())
+        .subscribe((people: Array<Object>) => this.people = people);
   }
 }