doc/filters/drawtext: fix centering example

Signed-off-by: Andrey Utkin <andrey.od.utkin@gmail.com>
Signed-off-by: Lou Logan <lou@lrcd.com>
(cherry picked from commit 648b26acc5)
Signed-off-by: Timothy Gu <timothygu99@gmail.com>

doc/filters.texi

@@ -4967,7 +4967,7 @@ within the parameter list.
 @item
 Show the text at the center of the video frame:
 @example
-drawtext="fontsize=30:fontfile=FreeSerif.ttf:text='hello world':x=(w-text_w)/2:y=(h-text_h-line_h)/2"
+drawtext="fontsize=30:fontfile=FreeSerif.ttf:text='hello world':x=(w-text_w)/2:y=(h-text_h)/2"
 @end example
 
 @item