fixed typo in material.py
fixed typo in material.py

liangjg/openmc,shikhar413/openmc,paulromano/openmc,shikhar413/openmc,liangjg/openmc,samuelshaner/openmc,walshjon/openmc,amandalund/openmc,walshjon/openmc,amandalund/openmc,walshjon/openmc,mit-crpg/openmc,johnnyliu27/openmc,smharper/openmc,shikhar413/openmc,smharper/openmc,liangjg/openmc,samuelshaner/openmc,johnnyliu27/openmc,mit-crpg/openmc,mit-crpg/openmc,mit-crpg/openmc,smharper/openmc,wbinventor/openmc,amandalund/openmc,bhermanmit/openmc,wbinventor/openmc,walshjon/openmc,johnnyliu27/openmc,wbinventor/openmc,liangjg/openmc,samuelshaner/openmc,bhermanmit/openmc,johnnyliu27/openmc,shikhar413/openmc,amandalund/openmc,paulromano/openmc,smharper/openmc,paulromano/openmc,paulromano/openmc,wbinventor/openmc,samuelshaner/openmc