gc docs: fix formatting for "gc.writeCommitGraph"

gc docs: fix formatting for "gc.writeCommitGraph"

Change the AsciiDoc formatting so that an example of "gc --auto" isn't
rendered as "git-gc(1) --auto", but as "git gc --auto". This is
consistent with the rest of the links and command examples in this
documentation.

The formatting I'm changing was initially introduced in
d5d5d7b641 ("gc: automatically write commit-graph files", 2018-06-27).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>