ntb.rst: Fix copy and paste error

It seems the text for the NTB MSI Test Client section was copied from the
NTB Tool Test Client, but was not updated for the new section.  Corrects
the NTB MSI Test Client section text.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Jon Mason <jdmason@kudzu.us>