Fix typo in documentation
Fix typo in documentation

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