midx: fix a formatting issue in "multi-pack-index.txt"

midx: fix a formatting issue in "multi-pack-index.txt"

There is a formatting issue  in "multi-pack-index.html", corresponding
to the nesting bulleted list of a wrong usage in "multi-pack-index.txt"
and this commit fix the problem.

In ASCIIDOC, it doesn't treat an indented character as the
beginning of a sub-list. If we want to write a nested bulleted list, we
could just use ASTERISK without any DASH like:

      "
      * Level 1 list item
      ** Level 2 list item
      *** Level 3 list item
      ** Level 2 list item
      * Level 1 list item
      ** Level 2 list item
      * Level 1 list item
      "

The DASH can be used for bulleted list too, But the DASH is suggested
only to be used as the marker for the first level because the DASH
doesn’t work well or a best practice for nested lists,
like (dash is as level 2 below):

      "
      * Level 1 list item
      - Level 2 list item
      * Level 1 list item
      "

ASTERISK is recommanded to use because it works intuitively and clearly
("marker length = nesting level") in nested lists, but the DASH can't.
However, when you want to write a non-nested bulleted lists, DASH works
too, like:

      "
      - Level 1 list item
      - Level 1 list item
      - Level 1 list item
      "

Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>