Fix comment.

src/common/observablemap.ts

@@ -350,7 +350,7 @@ class ObservableMap<T> implements IObservableMap<T> {
   }
 
   /**
-   * Catch a signal from the parent vector and pass it on.
+   * Catch a signal from the parent map and pass it on.
    */
   private _forwardSignal(s: IObservableMap<T>,
                          c: ObservableMap.IChangedArgs<T>) {