Fixed typo in filter.py
Fixed typo in filter.py

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