Fix event name

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>

proposals/3898-sfu.md

@@ -279,15 +279,15 @@ This event works exactly like the `m.call.negotiate` event in 1:1 calls.
 }
 ```
 
-##### `m.call.sdp_stream_metadata`
+##### `m.call.sdp_stream_metadata_changed`
 
-This event works very similarly to the 1:1 call `m.call.sdp_stream_metadata`.
+This event works very similarly to the 1:1 call `m.call.sdp_stream_metadata_changed`.
 
 - **TODO: Spec how foci actually use this to advertise tracks**
 
 ```json
 {
-    "type": "m.call.sdp_stream_metadata",
+    "type": "m.call.sdp_stream_metadata_changed",
     "content": {
         "sdp_stream_metadata": {...} // As specified in the Metadata section
     }