fix flvdec.c file description comment

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@6773 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/flvdec.c

 /*
- * FLV encoder.
+ * FLV decoder.
  * Copyright (c) 2003 The FFmpeg Project.
  *
  * This file is part of FFmpeg.