doc/git-repack: fix syntax for `-g` shorthand option

doc/git-repack: fix syntax for `-g` shorthand option

The `-g` switch is a shorthand for `--geometric=` and allows the user to
specify the geometric. The documentation is wrong though and indicates
that the syntax for the shorthand is `-g=<factor>`. In fact though, the
option must be specified without the equals sign via `-g<factor>`.

Fix the syntax accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>