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>