transcoding: fix Doxygen file path

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/examples/transcoding.c

@@ -25,7 +25,7 @@
 /**
  * @file
  * API example for demuxing, decoding, filtering, encoding and muxing
- * @example doc/examples/transcoding.c
+ * @example transcoding.c
  */
 
 #include <libavcodec/avcodec.h>