specs/qcow2: do not use wording 'bitmap header'

specs/qcow2: do not use wording 'bitmap header'

A bitmap directory entry is sometimes called a 'bitmap header'. This
patch leaves only one name - 'bitmap directory entry'. The name 'bitmap
header' creates misunderstandings with 'qcow2 header' and 'qcow2 bitmap
header extension' (which is extension of qcow2 header)

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20170628120530.31251-3-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>