Update security.md

Delete extra phrasing to match our tone.

Author: Amanda Butler

docs/api/security/security.md

@@ -8,4 +8,4 @@ Security on Arm Mbed OS is divided into the following parts:
 For information about working with these modules in Mbed OS context, please refer to the following document:
 
 - [Connection security through Arm Mbed TLS](tls.html).
-- [Mbed PSA asset protection and much much more](psa.md).
+- [Mbed PSA asset protection](psa.md).