doc/filters: correct ssim example

AVTB is 1/AV_TIME_BASE

doc/filters.texi

@@ -17136,7 +17136,7 @@ ffmpeg -i main.mpg -i ref.mpg -lavfi  "ssim;[0:v][1:v]psnr" -f null -
 @item
 Another example with 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]ssim" -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]ssim" -f null -
 @end example
 @end itemize