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>

Author: pks-t

Documentation/git-repack.txt

@@ -186,7 +186,7 @@ depth is 4095.
 	Pass the `--delta-islands` option to `git-pack-objects`, see
 	linkgit:git-pack-objects[1].
 
--g=<factor>::
+-g<factor>::
 --geometric=<factor>::
 	Arrange resulting pack structure so that each successive pack
 	contains at least `<factor>` times the number of objects as the