Page on keyboard input needs to have Input renamed to ButtonInput

[boreq]

Right now the page uses Input in the first example. It should be ButtonInput in v0.13 instead.

Old:

fn keyboard_input(
    keys: Res<Input<KeyCode>>,
) {

New:

fn keyboard_input(
    keys: Res<ButtonInput<KeyCode>>,
) {

I started preparing a pull request before I read that this isn't welcome, maybe this is useful to illustrate the problem.

boreq/bevy-cheatbook@main...boreq:keyboard-013