Update openmc/data/grid.py
Update openmc/data/grid.py

`Returns` -> `Return`

Co-Authored-By: Sterling Harper <9c609db483ef5a462d9ab2c3c8c3530e8f4b9007@gmail.com>
paulromano/openmc,shikhar413/openmc,liangjg/openmc,liangjg/openmc,walshjon/openmc,walshjon/openmc,walshjon/openmc,amandalund/openmc,smharper/openmc,walshjon/openmc,smharper/openmc,paulromano/openmc,amandalund/openmc,shikhar413/openmc,shikhar413/openmc,paulromano/openmc,mit-crpg/openmc,mit-crpg/openmc,liangjg/openmc,paulromano/openmc,amandalund/openmc,smharper/openmc,mit-crpg/openmc,smharper/openmc,mit-crpg/openmc,amandalund/openmc,liangjg/openmc,shikhar413/openmc