Fixes the staircases in the tutorial room.

Mentioned there: https://github.com/CleverRaven/Cataclysm-DDA/issues/9086#issuecomment-56455920

> I feel I need to mention something. There are a few other issues around the tutorial. Staircases are not functioning correctly in the tutorial (but despite that, the tutorial building has a downstairs that you can not access).

Fixed by this PR.

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> In addition to that, occasionally, a few tutorial messages mention something about a zombie upstairs for practicing combat, or

There are two messages that mention zombies: the information about butchering (zombies leave tainted meat) and this one:
`The item you just picked up is a good weapon!  To wield a weapon, press w, then pick what to wield.  To wield nothing, either drop your weapon with d, or press 'w-' to put it away.  A zombie has spawned nearby.  To attack it, simply move into it.` (LESSON_GUN_FIRE)

Nothing about upstairs here. However when you reload a gun, three zombies (the message only mentions one, but this has always been that way) spawn for you to practice firing. This does not depend upon the location (upper/lower level) of the character.

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> or "It's dark down here. You need to find a source of light. You can find a flashlight upstairs/downstairs".

The two messages in the tutorial that reference the dark are:

`It's dark down here!  Without a source of light, you'll be limited to seeing only one space in any direction.  You'll encounter darkness while underground or at night.  Press '<' to go back upstairs, and look around for a flashlight.` (LESSON_DARK_NO_FLASH)

`It's dark down here!  Fortunately, you have a flashlight.  Press 'a' and then select your flashlight to turn it on.` (LESSON_DARK)

Both messages are shown when the character movies into an area with low light, usually when going down into the basement (it could probably be triggered by waiting till dawn, but who does that).
The first message is shown when the character has no flashlight. The character is typically still on the stairs, so pressing '<' will indeed go up. There is always a flashlight in the ground level part of the tutorial (see mapgen_functions.cpp:6754).

The second message is shown when the character has flashlight, it is also correct.

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For reference: all tutorial messages are read from "data/json/tutorial.json".