Fix allowed Language Versions

ICSharpCode.ILSpyCmd/IlspyCmdProgram.cs

@@ -86,8 +86,8 @@ Examples:
 				typeof(FullTypeName).Assembly.GetName().Version.ToString();
 
 		[Option("-lv|--languageversion <version>", "C# Language version: CSharp1, CSharp2, CSharp3, " +
-			"CSharp4, CSharp5, CSharp6, CSharp7_0, CSharp7_1, CSharp7_2, CSharp7_3, CSharp8_0, CSharp9_0, " +
-			"CSharp_10_0 or Latest", CommandOptionType.SingleValue)]
+			"CSharp4, CSharp5, CSharp6, CSharp7, CSharp7_1, CSharp7_2, CSharp7_3, CSharp8_0, CSharp9_0, " +
+			"CSharp10_0, Preview or Latest", CommandOptionType.SingleValue)]
 		public LanguageVersion LanguageVersion { get; } = LanguageVersion.Latest;
 
 		[DirectoryExists]