Update archers_prelude.lua

Forgot to change the calculation. 
Source: http://wiki.bluegartr.com/bg/Archer%27s_Prelude
Ranged Accuracy bonus caps at +40 before enhancement.
    This requires approximately 600 combined skill. 
So calculation was 300+300 = 600 - 200 = 400 / 10 = 40 cap.