doc/decode_audio: use <> to include libav* headers

Found-by: ubitux

doc/examples/decode_audio.c

@@ -31,10 +31,10 @@
 #include <stdlib.h>
 #include <string.h>
 
-#include "libavutil/frame.h"
-#include "libavutil/mem.h"
+#include <libavutil/frame.h>
+#include <libavutil/mem.h>
 
-#include "libavcodec/avcodec.h"
+#include <libavcodec/avcodec.h>
 
 #define AUDIO_INBUF_SIZE 20480
 #define AUDIO_REFILL_THRESH 4096