(maint) Change required to true

In order to discard a release, we need to know which one it is, as such,
the Milestone option has to be included.

Author: gep13

Source/GitReleaseManager.Cli/Options/DiscardSubOptions.cs

@@ -11,7 +11,7 @@ namespace GitReleaseManager.Cli.Options
     [Verb("discard", HelpText = "Discards a draft release.")]
     public class DiscardSubOptions : BaseVcsOptions
     {
-        [Option('m', "milestone", HelpText = "The milestone to use.", Required = false)]
+        [Option('m', "milestone", HelpText = "The milestone to use.", Required = true)]
         public string Milestone { get; set; }
     }
-}
+}