matroskaenc: use correct EBML ID for block duration

Originally committed as revision 15560 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskaenc.c

@@ -778,7 +778,7 @@ static int mkv_write_packet(AVFormatContext *s, AVPacket *pkt)
         ebml_master blockgroup = start_ebml_master(pb, MATROSKA_ID_BLOCKGROUP, mkv_blockgroup_size(pkt->size));
         duration = pkt->convergence_duration;
         mkv_write_block(s, MATROSKA_ID_BLOCK, pkt, 0);
-        put_ebml_uint(pb, MATROSKA_ID_DURATION, duration);
+        put_ebml_uint(pb, MATROSKA_ID_BLOCKDURATION, duration);
         end_ebml_master(pb, blockgroup);
     }