documentation: git-update-index: add --show-index-version to synopsis

In 606e088 (update-index: add --show-index-version, 2023-09-12), we
added the new '--show-index-version' option to 'git-update-index' and
documented it, but forgot to add it to the synopsis section.

Add '--show-index-version' to the synopsis of 'git-update-index'.

Signed-off-by: Dov Murik <dov.murik@linux.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: dubek

Documentation/git-update-index.txt

@@ -25,6 +25,7 @@ SYNOPSIS
 	     [--really-refresh] [--unresolve] [--again | -g]
 	     [--info-only] [--index-info]
 	     [-z] [--stdin] [--index-version <n>]
+	     [--show-index-version]
 	     [--verbose]
 	     [--] [<file>...]