docs: correct typo

Author: Jennifer Bland

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

@@ -62,7 +62,7 @@ export interface MessageBusSource {
  attachToZone(zone: NgZone): void;
 
  /**
- * 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;