update the 3X15 rules

[lule75]

According to the new BWF experimental rules, on the prototype scoring system on the 3X15 (declared as 3X15_18), the maximum number of points per game is supposed to be 20. I noticed in the original source_code it stops on 18. Kindly update for everyone else. Thank you @phihag we Appreciate your efforts.

[phihag]

Thank you for creating this ticket! I will add a new scoring system for the new BWF experimental rules.