Two fixes to the leveling section of the startup guide

[jtdowney]

I was reconfiguring my Legacy and noticed the guide references https://www.klipper3d.org/Config_Reference.html#bed_tilt instead of https://www.klipper3d.org/Config_Reference.html#z_tilt. As far as I can tell from both the V1 and the Legacy firmware configuration, the correct one is z_tilt.

[shiftingtech]

thanks for that