doc/filters: add one more zoompan example

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -10149,6 +10149,12 @@ Zoom-in up to 1.5 and pan at same time to some spot near center of picture:
 @example
 zoompan=z='min(zoom+0.0015,1.5)':d=700:x='if(gte(zoom,1.5),x,x+1/a)':y='if(gte(zoom,1.5),y,y+1)':s=640x360
 @end example
+
+@item
+Zoom-in up to 1.5 and pan always at center of picture:
+@example
+zoompan=z='min(zoom+0.0015,1.5)':d=700:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'
+@end example
 @end itemize
 
 @c man end VIDEO FILTERS