fix inappropriate description for function in RSA

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index ba3a760..7b7c3e8 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -182,7 +182,8 @@
                             mbedtls_md_type_t hash_id);
 
 /**
- * \brief          This function retrieves padding mode of RSA modulus.
+ * \brief          This function retrieves padding mode of initialized
+ *                 RSA context.
  *
  * \param ctx      The initialized RSA context.
  *