Fixed parentheses in cross_section module.
Fixed parentheses in cross_section module.

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