fix x11grab example in e.g. the manpage so that they actually work

Originally committed as revision 25113 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -817,14 +817,14 @@ standard mixer.
 FFmpeg can grab the X11 display.
 
 @example
-ffmpeg -f x11grab -s cif -i :0.0 /tmp/out.mpg
+ffmpeg -f x11grab -s cif -r 25 -i :0.0 /tmp/out.mpg
 @end example
 
 0.0 is display.screen number of your X11 server, same as
 the DISPLAY environment variable.
 
 @example
-ffmpeg -f x11grab -s cif -i :0.0+10,20 /tmp/out.mpg
+ffmpeg -f x11grab -s cif -r 25 -i :0.0+10,20 /tmp/out.mpg
 @end example
 
 0.0 is display.screen number of your X11 server, same as the DISPLAY environment