rebase usage: subcommands can not be combined with -i

rebase usage: subcommands can not be combined with -i

Since 95135b0 (rebase: stricter check of standalone sub command,
2011-02-06), git-rebase has not allowed to use -i together with e.g.
--continue. Yet, when rebase started using OPTIONS_SPEC in 45e2acf
(rebase: define options in OPTIONS_SPEC, 2011-02-28), the usage message
included

	git-rebase [-i] --continue | --abort | --skip

Remove the "[-i]" from this line.

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>