fix(docs): remove misleading docplaster (#3085)

In this excerpt without the checkLogin method it might indicate to users that they have to create a new class.

Author: RicardoVaranda

public/docs/_examples/router/ts/app/auth-guard.service.3.ts

@@ -1,4 +1,3 @@
-// #docplaster
 // #docregion
 // #docregion can-activate-child
 import { Injectable }       from '@angular/core';