git.txt: fix synopsis of `--config-env` missing the equals sign

When executing `git -h`, then the `--config-env` documentation rightly lists the option as requiring an equals between the option and its argument: this is the only currently supported format. But the git(1) manpage incorrectly lists the option as taking a space in between. Fix the issue by adding the missing space. Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-of-by: Patrick Steinhardt <ps@pks.im> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>