docs: fix missed mention of duration

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

man/picom.1.adoc

@@ -745,7 +745,7 @@ Currently, these context variables are defined: :::
 
   _window-raw-opacity-before_, _window-raw-opacity_:: Animation triggers are usually accompanied by a change in the window's opacity. For example, when a window is opened, its opacity changes from 0 to 1. These two variables reflect the opacity of the window for the previous and current frame. They are useful if you want to smoothly transition the window's opacity.
 
-IMPORTANT: All of the _window-*-before_ variables are updated every frame, and reflects the state of the window in the previous frame. Which means they will only be meaningful for a single frame, when an animation has just been triggered. Which means you should only use them to define the _start_, _end_, or _delay_ values of a timing function, since these values are only evaluated once when the animation starts.
+IMPORTANT: All of the _window-*-before_ variables are updated every frame, and reflects the state of the window in the previous frame. Which means they will only be meaningful for a single frame, when an animation has just been triggered. Which means you should only use them to define the _start_, _end_, _duration_, or _delay_ values of a timing function, since these values are only evaluated once when the animation starts.
 --
 
 === Share your animations