video_frame_metadata: use more specific rtp timestamp spec reference

BUG=None

Change-Id: I6941d9b5ecf5715297fe794717d6a649521e4b1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627402Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843690}

media/base/video_frame_metadata.h

@@ -165,7 +165,7 @@ struct MEDIA_EXPORT VideoFrameMetadata {
   // The RTP timestamp associated with this video frame. Stored as a double
   // since base::DictionaryValue doesn't have a uint32_t type.
   //
-  // https://w3c.github.io/webrtc-pc/#dom-rtcrtpcontributingsource
+  // https://w3c.github.io/webrtc-pc/#dom-rtcrtpcontributingsource-rtptimestamp
   base::Optional<double> rtp_timestamp;
 
   // For video frames coming from a remote source, this is the time the