rev-list-options.txt: fix rendering of bonus paragraph

rev-list-options.txt: fix rendering of bonus paragraph

In git-log(1) -- but not in git-shortlog(1) or git-rev-list(1) -- we
include a bonus paragraph in the description of `--first-parent`. But
we forgot to add a lone "+" for a list continuation, and we shouldn't
be indenting this second paragraph. As a result, we get a different
indentation and the `backticks` render literally.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>