fix "than" typo

Found-by reimar
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2330,7 +2330,7 @@ static void estimate_timings_from_bit_rate(AVFormatContext *ic)
                 bit_rate += st->codec->bit_rate;
             } else if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO && st->codec_info_nb_frames > 1) {
                 // If we have a videostream with packets but without a bitrate
-                // than consider the sum not known
+                // then consider the sum not known
                 bit_rate = 0;
                 break;
             }