fix "syntax error in expression"

AAXtoMP3

@@ -801,7 +801,7 @@ do
 
   extra_crop_cover=''
   cover_width=$(ffprobe -i "${cover_file}" 2>&1 | grep -Po "[0-9]+(?=x[0-9]+)")
-  if [ "$(( ${cover_width} % 2 ))" == "1" ]; then
+  if (( ${cover_width} % 2 == 1 )); then
     if [ "$((${loglevel} > 1))" == "1" ]; then
       log "Cover ${cover_file} has odd width ${cover_width}, setting extra_crop_cover to make even."
     fi