DOC: correct label statistics bounding box definition

Change-Id: I58a709c0f68be897dd594012f3c5f3a4a287628a
DOC: update description of label statistics image filter

Change-Id: I806e0796d820fcb01c56896ead8429559fcddb13
Merge topic 'SIMPLEITK-421_FixStatisticsDoc' into next

83b72f3 DOC: update description of label statistics image filter
f23a9aa DOC: correct label statistics bounding box definition