Update 'linux/ffmpeg.md'

linux/ffmpeg.md

@@ -1,16 +1,16 @@
  ## Duplicate DTS and encode AAC
- '''
+ ```
  ffmpeg -y -i <input> -strict -2 -map 0 -c copy -map 0:a0 -c:a:1 aac -b:a 192k -ac 2 -f matroska <out>
- '''
+ ```
 
   ## Attach Cover Art
- '''
+ ```
  function addcover(){
  ffmpeg -i $1.mkv -map 0 -c copy \
         -attach $1-poster.jpg -metadata:s:t mimetype=image/jpeg -metadata:s:t filename=cover.jpg \
         -attach $1-fanart.jpg -metadata:s:t mimetype=image/jpeg -metadata:s:t filename=fanart.jpg \
         $1.plus.mkv
  }
- '''
+ ```
  Note: gerbera doesn't read the jpg. :(
      Why?