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>

lib/Kconfig

@@ -296,11 +296,9 @@ config MPILIB_EXTRA
 	bool "Multiprecision maths library - additional sources"
 	depends on MPILIB
 	help
-	  Multiprecision maths library from GnuPG.
-	  It is used to implement RSA digital signature verification,
-	  which is used by IMA/EVM digital signature extension.
-	  This code in unnecessary for RSA digital signature verification,
-	  and can be compiled if needed.
+	  Additional sources of multiprecision maths library from GnuPG.
+	  This code is unnecessary for RSA digital signature verification,
+	  but can be compiled if needed.
 
 config SIGNATURE
 	tristate "In-kernel signature checker"