qapi: Fix unintended definition lists in documentation

rST parses something like

    first line
        second line

as a definition list item, where "first line" is the term being
defined by "second line".

This bites us in a couple of places.  Here's one:

    # @bps_max: total throughput limit during bursts,
    #                     in bytes (Since 1.7)

scripts/qapi/parser.py parses this into an "argument section" with
name "bps_max" and text

    total throughput limit during bursts,
	      in bytes (Since 1.7)

docs/sphinx/qapidoc.py duly passes the text to the rST parser, which
parses it as another definition list.  Comes out as nested
definitions: term "bps_max: int (optional)" defined as term "total
throughput limit during bursts," defined as "in bytes (Since 1.7)".

rST truly is the Perl of ASCII-based markups.

Fix by deleting the extra indentation.

Fixes: 26ec4e53 (qapi: Fix indent level on doc comments in json files)
Fixes: c0ac533b (qapi: Stop using whitespace for alignment in comments)
Fixes: 81ad2964 (net/vmnet: add vmnet backends to qapi/net)
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230425064223.820979-11-armbru@redhat.com>