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>