Subtract the margin from the value returned by ProbCut

Subtract the margin from the value returned by ProbCut

This patch subtracts the margin from the value returned by ProbCut.

Passed STC:
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 46112 W: 11940 L: 11610 D: 22562
Ptnml(0-2): 131, 5318, 11842, 5620, 145
https://tests.stockfishchess.org/tests/view/652ea42ade6d262d08d329dd

Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 86880 W: 22192 L: 21776 D: 42912
Ptnml(0-2): 43, 9213, 24510, 9633, 41
https://tests.stockfishchess.org/tests/view/652f70ffde6d262d08d33e8d

closes https://github.com/official-stockfish/Stockfish/pull/4835

bench: 1135313