mariabackup : improve help text

extra/mariabackup/xtrabackup.cc

@@ -1213,7 +1213,7 @@ GNU General Public License for more details.\n\
 \n\
 You can download full text of the license on http://www.gnu.org/licenses/gpl-2.0.txt\n");
 
-  printf("Usage: [%s [--defaults-file=#] --backup | %s [--defaults-file=#] --prepare] [OPTIONS]\n",my_progname,my_progname);
+  printf("Usage: %s [--defaults-file=#] [--backup | --prepare | --copy-back | --move-back] [OPTIONS]\n",my_progname);
   print_defaults("my", xb_server_default_groups);
   my_print_help(xb_client_options);
   my_print_help(xb_server_options);