Fix the defense evaluation

[programarivm]

The defense evaluation in src/Eval/DefenseEval.php is still perhaps not too clear in its explanation and needs some fine-tuning as described in the following example.

If the pawn on a7 is moved, these pieces will be exposed to attack: White's queen on a4, The rook on a8:

• Movetext: 1.c4 e6 2.Nc3 d5 3.cxd5 exd5 4.d4 Bb4 5.Bd2 c5 6.Qa4+ Bd7 7.Nb5 Bxd2+ 8.Kxd2 Qb6 9.e3 c4

• FEN: rn2k1nr/pp1b1ppp/1q6/1N1p4/Q1pP4/4P3/PP1K1PPP/R4BNR w kq -

Happy learning!