Improve Materials.export_to_xml comment
Improve Materials.export_to_xml comment

liangjg/openmc,paulromano/openmc,amandalund/openmc,mit-crpg/openmc,paulromano/openmc,amandalund/openmc,liangjg/openmc,walshjon/openmc,paulromano/openmc,shikhar413/openmc,amandalund/openmc,smharper/openmc,smharper/openmc,walshjon/openmc,smharper/openmc,mit-crpg/openmc,liangjg/openmc,shikhar413/openmc,paulromano/openmc,walshjon/openmc,walshjon/openmc,liangjg/openmc,shikhar413/openmc,mit-crpg/openmc,smharper/openmc,shikhar413/openmc,amandalund/openmc,mit-crpg/openmc