Fix preview option (#58)

Author: josesimoes

nanoFirmwareFlasher/Options.cs

@@ -231,7 +231,7 @@ public class Options
         public bool Deploy { get; set; }
 
         [Option(
-            "stable",
+            "preview",
             Required = false,
             Default = false,
             HelpText = "Preview version. Will download the firmware package from the preview repository.")]