avcodec: fix AVpacket → AVPacket typo.

libavcodec/avcodec.h

@@ -3835,7 +3835,7 @@ int avcodec_decode_audio4(AVCodecContext *avctx, AVFrame *frame,
  *             next call to this function or until closing the decoder. The
  *             caller may not write to it.
  *
- * @param[in] avpkt The input AVpacket containing the input buffer.
+ * @param[in] avpkt The input AVPacket containing the input buffer.
  *            You can create such packet with av_init_packet() and by then setting
  *            data and size, some decoders might in addition need other fields like
  *            flags&AV_PKT_FLAG_KEY. All decoders are designed to use the least