Documentation: git-archive: mark --format as optional in summary

The --format option was made optional in 8ff21b1 (git-archive: make
tar the default format, 2007-04-09), but it was not marked as optional
in the summary. This trival patch just changes the summary to match
the rest of the documentation.

Signed-off-by: Wesley J. Landaker <wjl@icecavern.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-archive.txt

@@ -9,7 +9,7 @@ git-archive - Create an archive of files from a named tree
 SYNOPSIS
 --------
 [verse]
-'git archive' --format=<fmt> [--list] [--prefix=<prefix>/] [<extra>]
+'git archive' [--format=<fmt>] [--list] [--prefix=<prefix>/] [<extra>]
 	      [--remote=<repo> [--exec=<git-upload-archive>]] <tree-ish>
 	      [path...]