Remove TBD line.

Remove line that doesn't include helpful information. We can replace it once we have a link and content.

Author: Amanda Butler

docs/reference/api/security/security.md

@@ -5,7 +5,6 @@ Security on Arm Mbed OS is divided into the following parts:
 - Mbed TLS. For details, please see our [full documentation](https://tls.mbed.org/).
 - Platform Security Architecture (PSA). For details, please see our [full documentation](https://developer.arm.com/products/architecture/security-architectures/platform-security-architecture).
 
-For information about working with these modules in Mbed OS context, please refer to the following documents:
+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).
-- Platform Security Architecture - TBD - under development.