sdp: fix opus sprop-stereo fmtp syntax

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/sdp.c

@@ -692,7 +692,7 @@ static char *sdp_write_media_attributes(char *buff, int size, AVCodecParameters
             av_strlcatf(buff, size, "a=rtpmap:%d opus/48000/2\r\n",
                                      payload_type);
             if (p->channels == 2) {
-                av_strlcatf(buff, size, "a=fmtp:%d sprop-stereo:1\r\n",
+                av_strlcatf(buff, size, "a=fmtp:%d sprop-stereo=1\r\n",
                                          payload_type);
             }
             break;