Adjust comment describing mbedtls_ssl_conf_sni()

Signed-off-by: Glenn Strauss <gstrauss@gluelogic.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 738f07b..aa6a212 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -3624,8 +3624,7 @@
  *                 mbedtls_ssl_set_hs_ca_chain() as well as the client
  *                 authentication mode with \c mbedtls_ssl_set_hs_authmode(),
  *                 then must return 0. If no matching name is found, the
- *                 callback must either set a default cert, or
- *                 return non-zero to abort the handshake at this point.
+ *                 callback may return non-zero to abort the handshake.
  *
  * \param conf     SSL configuration
  * \param f_sni    verification function