Tweak piece values of silver and gold

Tweak piece values of silver and gold

minishogi
LLR: 2.95 (-2.94,2.94) [0.00,10.00]
Total: 1794 W: 958 L: 830 D: 6

makruk STC
LLR: 2.98 (-2.94,2.94) [-10.00,5.00]
Total: 844 W: 99 L: 63 D: 682
http://35.161.250.236:6543/tests/view/5d66db396e23db3768ec07a8

makruk LTC
LLR: 3.00 (-2.94,2.94) [-10.00,5.00]
Total: 1269 W: 115 L: 80 D: 1074
http://35.161.250.236:6543/tests/view/5d676dfd6e23db3768ec07ae

sittuyin STC
LLR: 3.00 (-2.94,2.94) [-10.00,5.00]
Total: 1150 W: 206 L: 169 D: 775
http://35.161.250.236:6543/tests/view/5d66db566e23db3768ec07ac