Merge pull request #133 from w3c/fix-hdrext

Replace "a=hdrext" with "a=extmap"

index.html

@@ -157,7 +157,7 @@ <h2>Modifications to existing procedures</h2>
         right after the step that sets transceiver.[[\Sender]].[[\SendCodecs]],
         saying "For each transciever, set {{RTCRtpTransceiver/[[HeaderExtensionsNegotiated]]}} to
         an empty list, and then
-        for each <code>"a=hdrext"</code> line, add an appropriate
+        for each <code>"a=extmap"</code> line, add an appropriate
         {{RTCRtpHeaderExtensionCapability}} to the list.
       </p>
       <p>