add zstd to usage

volume-backup.sh

@@ -4,7 +4,7 @@ usage() {
   >&2 echo "Usage: volume-backup <backup|restore> [options] <archive or - for stdin/stdout>"
   >&2 echo ""
   >&2 echo "Options:"
-  >&2 echo "  -c <algorithm> chooose compression algorithm: bz2 (default), gz, xz and 0 (none)"
+  >&2 echo "  -c <algorithm> chooose compression algorithm: bz2 (default), gz, xz, zstd and 0 (none)"
   >&2 echo "  -e <glob> exclude files or directories (only for backup operation)"
   >&2 echo "  -v verbose"
 }