Fix typo in units description
Fix typo in units description

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