Fix typo in write_initial description.
Fix typo in write_initial description.

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