Fix drawer slide orientation suggestion

[SriramKalki]

Is it really true that mounting slides horizontally is unrecommended? (From Henopied)

[abidingabi]

I think horizontally here means the orientation of the actual slides, not which way they're extended; with e.g. these being horizontal: https://gm0.org/en/latest/_images/7236-cable-carrier.png, as opposed to how they are mounted in a drawer (facing each other).