docs(web_workers/shared/message_bus.ts): correct typo 'messsage' to 'message'

Corrects the typo 'messsage' to 'message' on line 35. Closes angular#4085

Author: rkho

modules/angular2/src/web_workers/shared/message_bus.ts

@@ -32,7 +32,7 @@ export /* abstract (with TS 1.6) */ class MessageBus implements MessageBusSource
  attachToZone(zone: NgZone): void { throw _abstract(); }
 
  /**
- * Returns an {@link EventEmitter} that emits every time a messsage
+ * Returns an {@link EventEmitter} that emits every time a message
  * is received on the given channel.
  */
  from(channel: string): EventEmitter { throw _abstract(); }