fix format and comment issues

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_keys.h b/library/ssl_tls13_keys.h
index c5baf28..34c00a4 100644
--- a/library/ssl_tls13_keys.h
+++ b/library/ssl_tls13_keys.h
@@ -693,12 +693,12 @@
 int mbedtls_ssl_tls13_compute_application_transform( mbedtls_ssl_context *ssl );
 
 /**
- * \brief Export TLS 1.3 PSK key from handshake context
+ * \brief Export TLS 1.3 PSK from handshake context
  *
  * \param ssl  The SSL context to operate on.
- * \param psk  PSK key output pointer.
+ * \param psk  PSK output pointer.
  * \param psk_len
- *             Length of PSK key.
+ *             Length of PSK.
  *
  * \returns    \c 0 on success.
  * \returns    A negative error code on failure.