qapi/block-core: Tidy up BlockLatencyHistogramInfo documentation

Documentation for member @bin comes out like

    list of io request counts corresponding to histogram intervals.
    len("bins") = len("boundaries") + 1 For the example above, "bins"
    may be something like [3, 1, 5, 2], and corresponding histogram
    looks like:

Note how the equation and the sentence following it run together.
Replace the equation:

    list of io request counts corresponding to histogram intervals,
    one more element than "boundaries" has.  For the example above,
    "bins" may be something like [3, 1, 5, 2], and corresponding
    histogram looks like:

Cc: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20230720071610.1096458-2-armbru@redhat.com>
[Off by one fixed]