Improve ffmpeg encoding instructions.

  - Applies suggestion from a comment by rafalfaro18 (
#20 (comment)).
  - Part of #20. Leave issue open, because it still assumes a single audio element.

PiperOrigin-RevId: 713359902

docs/external/encoding_with_external_tools.md

@@ -219,7 +219,7 @@ done
 ffmpeg \
   -i $input_iamf \
   -i $input_video \
-  -c:a copy -map 0:a:0 \
+  -c:a copy -map 0:a \
   -c:v copy -map 1:v:0 \
   -stream_group map=0=0$st_map \
   -stream_group map=0=1$st_map \