Do not mis-spell the `git pack-objects` command

Even in translations, the command names need to refer to the actual
Git command whose name is not to be translated.

This was reported by Todd Zullinger as a broken link to the manual page
in https://lore.kernel.org/git/ZWVIRXodL9pQZbtj@pobox.com/

.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>