Fix misleading LEVEL_3_MIN field

[Chocohead]

LEVEL_3 is 1 - 3 (e.g. non-empty cauldrons), so a constant field of zero won't be its minimum.
LEVEL_8 is 0 - 8 (e.g. composters), and is just after, so field_31387 is more likely to be for that.

It does leave field_31388 immediately underneath in a slightly awkward position, as it's a constant of one, so could either be LEVEL_1_8_MIN (as it is now) or LEVEL_3_MIN (to match field_31389 immediate after it).
I suppose field_31390 is in a similar position being a constant of eight, it could be LEVEL_1_8_MAX (as it is now) or LEVEL_8_MAX (to correspond to LEVEL_8_MIN).