i40e: fix a typo in i40e_pf documentation

This patch fixes a typo in i40e_pf object documentation; num_req_vfs
refers to the number of VFs requested for the PF.

Signed-off-by: Rami Rosen <rami.rosen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>