fix: Add output format for the pipe() example fluent-ffmpeg#1277 by @lin826 (#12)

Co-authored-by: I-Ju Lin <9304486+lin826@users.noreply.github.com>

Author: Jonham

README.md

@@ -937,6 +937,7 @@ var command = ffmpeg('/path/to/file.avi')
   .videoCodec('libx264')
   .audioCodec('libmp3lame')
   .size('320x240')
+  .format('flv')
   .on('error', function(err) {
     console.log('An error occurred: ' + err.message);
   })