Fix some metastables in cross_sections_serpent.xml.
Fix some metastables in cross_sections_serpent.xml.

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