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>

Documentation/git.txt

@@ -13,7 +13,7 @@ SYNOPSIS
     [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
     [-p|--paginate|-P|--no-pager] [--no-replace-objects] [--bare]
     [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
-    [--super-prefix=<path>] [--config-env <name>=<envvar>]
+    [--super-prefix=<path>] [--config-env=<name>=<envvar>]
     <command> [<args>]
 
 DESCRIPTION