Kernel joystick_get IRQ handler example is out of date

https://github.com/X16Community/x16-docs/blob/master/X16%20Reference%20-%2005%20-%20KERNAL.md#function-name-joystick_get

The IRQ handler isn't correct you now need to add the code below before the call to `SCNKEY`
```
    lda #08
    jsr $feab
```
