[observatory] Use month numbers ranging from 1-12 in the timeline default filename

Change-Id: I6184efa1e4d7d64e3f0fc4def14587898d29601a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/128983
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Jason Simmons <jsimmons@google.com>

runtime/observatory/web/timeline.js

@@ -591,7 +591,7 @@ function saveTimeline() {
   var defaultFilename = 'dart-timeline-' +
                         now.getFullYear() +
                         '-' +
-                        now.getMonth() +
+                        (now.getMonth() + 1) +
                         '-' +
                         now.getDate() +
                         '.json';