Singular extension search tweak

Singular extension search tweak

Tweak depth.

STC https://tests.stockfishchess.org/tests/view/5f2d22ec61e3b6af64881f40
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 17984 W: 2603 L: 2441 D: 12940
Ptnml(0-2): 133, 1751, 5094, 1849, 165

LTC https://tests.stockfishchess.org/tests/view/5f2d5a6a61e3b6af64881f7f
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 85808 W: 5956 L: 5621 D: 74231
Ptnml(0-2): 149, 4748, 32785, 5063, 159

closes https://github.com/official-stockfish/Stockfish/pull/2950

fixes two README.md typos:
fixes https://github.com/official-stockfish/Stockfish/issues/2932

bench: 4022669