Improve command line help

Fixes #450

src/main/java/org/asamk/signal/commands/RegisterCommand.java

@@ -16,8 +16,9 @@ public class RegisterCommand implements RegistrationCommand {
 
     @Override
     public void attachToSubparser(final Subparser subparser) {
+        subparser.help("Register a phone number with SMS or voice verification.");
         subparser.addArgument("-v", "--voice")
-                .help("The verification should be done over voice, not sms.")
+                .help("The verification should be done over voice, not SMS.")
                 .action(Arguments.storeTrue());
         subparser.addArgument("--captcha")
                 .help("The captcha token, required if registration failed with a captcha required error.");