Fix RBY grass/water encounter slot rarities.

Fix RBY grass/water encounter slot rarities.

<http://tasvideos.org/PokemonTricks.html>
This page explains how the RNG in RBY works, and gives exact rarities.
They are out of 256 rather than 100, so some rounding is necessary to
fit them into the database.

====  =======  ======  ==========  =======
Slot   Range   Chance  Percentage  Rounded
====  =======  ======  ==========  =======
0       0-50   51/256  19.921875%    20%
1      51-101  51/256  19.921875%    20%
2     102-140  39/256  15.234375%    15%
3     141-165  25/256   9.765625%    10%
4     166-190  25/256   9.765625%    10%
5     191-215  25/256   9.765625%    10%
6     216-228  13/256   5.078125%     5%
7     229-241  13/256   5.078125%     5%
8     242-252  11/256   4.296875%     4%
9     253-255   3/256   1.171875%     1%
====  =======  ======  ==========  =======