DOC: updated comment for ITK_FUTURE_LEGACY_REMOVE
DOC: updated comment for ITK_FUTURE_LEGACY_REMOVE

Changed ITKv5 to ITKv6.

InsightSoftwareConsortium/ITK,hjmjohnson/ITK,richardbeare/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,hjmjohnson/ITK,richardbeare/ITK,richardbeare/ITK,richardbeare/ITK,hjmjohnson/ITK,hjmjohnson/ITK,richardbeare/ITK,Kitware/ITK,Kitware/ITK,richardbeare/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,hjmjohnson/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK