avcodec/internal: update FF_CODEC_CAP_SETS_PKT_DTS doxy

The code it refers to was moved to decode.c in 00fb745a

libavcodec/internal.h

@@ -48,8 +48,8 @@
 #define FF_CODEC_CAP_INIT_CLEANUP           (1 << 1)
 /**
  * Decoders marked with FF_CODEC_CAP_SETS_PKT_DTS want to set
- * AVFrame.pkt_dts manually. If the flag is set, utils.c won't overwrite
- * this field. If it's unset, utils.c tries to guess the pkt_dts field
+ * AVFrame.pkt_dts manually. If the flag is set, decode.c won't overwrite
+ * this field. If it's unset, decode.c tries to guess the pkt_dts field
  * from the input AVPacket.
  */
 #define FF_CODEC_CAP_SETS_PKT_DTS           (1 << 2)