feat: 🎸 gem の description を更新した

String クラス についての記述がなかったので追加した

prekin.gemspec

@@ -9,7 +9,7 @@ Gem::Specification.new do |spec|
   spec.email         = ['takiya@toran.sakura.ne.jp']
 
   spec.summary       = "Judge whether the target day of month is 'PREMIUM FRIDAY' or not."
-  spec.description   = "Return true or false when you specify the day of month (by Time Class, Date Class or DateTime Class). The criteria is whether the day is 'PREMIUM FRIDAY' or not. 'PREMIUM FRIDAY' is the last friday in month in Japan."
+  spec.description   = "Return true or false when you specify the day of month (by Time class, Date class, DateTime class or String class). The criteria is whether the day is 'PREMIUM FRIDAY' or not. 'PREMIUM FRIDAY' is the last friday in month in Japan."
   spec.homepage      = 'https://github.com/nikukyugamer/prekin'
   spec.license       = 'MIT'
   spec.files         = Dir.chdir(File.expand_path(__dir__)) do