gc docs: change --keep-base-pack to --keep-largest-pack

gc docs: change --keep-base-pack to --keep-largest-pack

The --keep-base-pack option never existed in git.git. It was the name
for the --keep-largest-pack option in earlier revisions of that series
before it landed as ae4e89e549 ("gc: add --keep-largest-pack option",
2018-04-15).

The later patches in that series[1][2] weren't changed to also refer
to --keep-largest-pack, so we've had this reference to a nonexisting
option ever since the feature initially landed.

1. 55dfe13df9 ("gc: add gc.bigPackThreshold config", 2018-04-15)

2. 9806f5a7bf ("gc --auto: exclude base pack if not enough mem to
   "repack -ad"", 2018-04-15)

Reported-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>