docs(cookbook/graphql): fixed lint errors

Author: Urigo

public/docs/_examples/heroes-graphql/ts/app/hero-search.component.ts

@@ -4,7 +4,6 @@ import { Component, OnInit } from '@angular/core';
 import { Router }            from '@angular/router';
 
 // #docregion rxjs-imports
-import { Observable }        from 'rxjs/Observable';
 import { Subject }           from 'rxjs/Subject';
 
 // Observable class extensions