Tweak rfp margin in improving nodes

[TheRealGioviok]

Passed STC:

Test  | improvingrfp2
Elo   | 18.20 +- 6.24 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 3.01 (-2.94, 2.94) [0.00, 3.00]
Games | N: 4166 W: 1204 L: 986 D: 1976
Penta | [38, 447, 920, 615, 63]

https://ob.cwchess.org/test/1508/

Passed LTC:

Test  | improvingrfp2
Elo   | 1.74 +- 1.40 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=64MB
LLR   | 2.95 (-2.94, 2.94) [0.00, 3.00]
Games | N: 66766 W: 16662 L: 16328 D: 33776
Penta | [349, 7808, 16733, 8146, 347]

https://ob.cwchess.org/test/1509/

Bench: 12898941