Correct Return Type That Closure Missed

It appears that closure did not notice that a return type
was not consistent. This corrects the return type to match
what the download method in download manager actually
returns.

Issue #1248

Change-Id: I796e31d9e7ad1d812bd53a55f5f1c745ac0b187c

lib/offline/download_manager.js

@@ -141,7 +141,7 @@ shaka.offline.DownloadManager.prototype.queue = function(
 
 /**
  * @param {!shaka.net.NetworkingEngine} net
- * @return {!Promise.<number>}
+ * @return {!Promise}
  */
 shaka.offline.DownloadManager.prototype.download = function(net) {
   let groups = shaka.util.MapUtils.values(this.groups_);