Fix reference to timestamp definition

RFC3550 instead of 3559
close #2281

webrtc.html

@@ -7738,7 +7738,7 @@ <h2>Dictionary RTCRtpContributingSource Members</h2>
             <dt data-tests="RTCRtpReceiver-getSynchronizationSources.https.html"><dfn data-idl><code>rtpTimestamp</code></dfn> of type
                 <span class="idlMemberType">unsigned long</span>, required</dt>
             <dd>
-              <p>The last RTP timestamp, as defined in [[!RFC3559]] Section 5.1,
+              <p>The last RTP timestamp, as defined in [[!RFC3550]] Section 5.1,
               of the media played out at <var>timestamp</var>.</p>
             </dd>
           </dl>