One method, xsk_umem_setup, had an incorrect kernel doc
description, which has been corrected.

Also fixes small typos found in the comments.
Signed-off-by: NJan Sokolowski <jan.sokolowski@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>