pad: fix example explanation

"row" and "column" term positions in the explanation were inverted.
Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>

doc/filters.texi

@@ -984,7 +984,7 @@ For example:
 @example
 # Add paddings with color "violet" to the input video. Output video
 # size is 640x480, the top-left corner of the input video is placed at
-# row 0, column 40.
+# column 0, row 40.
 pad=640:480:0:40:violet
 @end example