doc/filters: correct libvmaf example

AVTB is 1/AV_TIME_BASE

doc/filters.texi

@@ -12448,7 +12448,7 @@ ffmpeg -i main.mpg -i ref.mpg -lavfi libvmaf="psnr=1:log_fmt=json" -f null -
 @item
 Example with options and different containers:
 @example
-ffmpeg -i main.mpg -i ref.mkv -lavfi "[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]libvmaf=psnr=1:log_fmt=json" -f null -
+ffmpeg -i main.mpg -i ref.mkv -lavfi "[0:v]settb=AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=AVTB,setpts=PTS-STARTPTS[ref];[main][ref]libvmaf=psnr=1:log_fmt=json" -f null -
 @end example
 @end itemize