Fixed typos in rectangular prism docstring per comments by @paulromano
Fixed typos in rectangular prism docstring per comments by @paulromano

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