docs: mention the "new" page up/down control

These were added a while ago, but I forgot to update the docs.

Author: Tygo-van-den-Hurk

docs/input-bindings.md

@@ -4,11 +4,11 @@ Slyde uses keybindings, and mouse events to make your life easier. I tried to st
 
 ## Next Slide
 
-Press one of the following keys to go to the next slide: `Enter`, `Space`, `d`, `l`, and `ArrowRight`. As well as pressing the primary mouse button, or scrolling down.
+Press one of the following keys to go to the next slide: `Enter`, `Space`, `d`, `l`, `PageUp`, and `ArrowRight`. As well as pressing the primary mouse button, or scrolling down.
 
 ## Previous Slide
 
-Press one of the following keys to go to the previous slide: `a`, `h`, and `ArrowLeft`. As well as pressing the secondary mouse button, or scrolling up.
+Press one of the following keys to go to the previous slide: `a`, `h`, `PageDown`, and `ArrowLeft`. As well as pressing the secondary mouse button, or scrolling up.
 
 ## Full Screen