ヘルプメッセージのTypoを修正

[ysakasin]

Fix #390

[codecov]

Codecov Report

Merging #391 (b54e218) into master (b893679) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #391   +/-   ##
=======================================
  Coverage   94.56%   94.56%           
=======================================
  Files         275      275           
  Lines       18636    18636           
=======================================
  Hits        17624    17624           
  Misses       1012     1012           

Impacted Files	Coverage Δ
lib/bcdice/game_system/CardRanker.rb	100.00% <ø> (ø)
lib/bcdice/game_system/Garako.rb	98.46% <ø> (ø)
lib/bcdice/game_system/LostRoyal.rb	100.00% <ø> (ø)
lib/bcdice/game_system/Skynauts.rb	99.40% <ø> (ø)
lib/bcdice/game_system/Warhammer4.rb	96.03% <ø> (ø)
lib/bcdice/game_system/Torg.rb	96.07% <100.00%> (ø)
lib/bcdice/game_system/TorgEternity.rb	99.10% <100.00%> (ø)

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