lib: MPILIB Kconfig description update

It was reported that description of the MPILIB_EXTRA is confusing.
Indeed it was copy-paste typo. It is fixed here.

Reported-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: James Morris <jmorris@namei.org>