[Cthulhu] RESx-yを実行できないように

[ysakasin]

RESx-y が実行できるが、 RESx で解釈されていた。

ヘルプには RES(x-y) が想定書式なので、 RESx-y は実行しないようにする。

[codecov]

Codecov Report

Merging #521 (117d18a) into master (c198700) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #521   +/-   ##
=======================================
  Coverage   95.53%   95.54%           
=======================================
  Files         319      319           
  Lines       18736    18736           
=======================================
+ Hits        17900    17901    +1     
+ Misses        836      835    -1     

Impacted Files	Coverage Δ
lib/bcdice/game_system/Cthulhu.rb	98.50% <100.00%> (+0.74%)	⬆️

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