Update rsa.h
minor change to the file's brief desc.
/**
* \file rsa.h
*
- * \brief This file defines the RSA public-key cryptosystem.
+ * \brief This file provides an API for the RSA public-key cryptosystem.
*
* The RSA public-key cryptosystem is defined in <em>Public-Key
* Cryptography Standards (PKCS) #1 v1.5: RSA Encryption</em>
