containerlab: Change not in user group hint to user usermod instead o…

…f gpasswd

cmd/root.go

@@ -104,7 +104,7 @@ func checkAndGetRootPrivs(_ *cobra.Command, _ []string) error {
 			}
 
 			if !slices.Contains(effUserGroupIDs, clabGroup.Gid) {
-				return fmt.Errorf("user '%v' is not part of containerlab admin group 'clab_admins' (GID %v), which is required to execute this command.\nTo add yourself to this group, run the following command:\n\t$ sudo gpasswd -a %v clab_admins",
+				return fmt.Errorf("user '%v' is not part of containerlab admin group 'clab_admins' (GID %v), which is required to execute this command.\nTo add yourself to this group, run the following command:\n\t$ sudo usermod -aG clab_admins %v",
 					currentEffUser.Username, clabGroup.Gid, currentEffUser.Username)
 			}