Fixed typos in physics section in documentation.
Fixed typos in physics section in documentation.

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