Rename ssl_conf_datagram_packing() to ssl_set_datagram_packing()

The naming convention is that functions of the form mbedtls_ssl_conf_xxx()
apply to the SSL configuration.
diff --git a/ChangeLog b/ChangeLog
index 4bf4c8e..8f05896 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -14,7 +14,7 @@
      enabled by default.
 
 API Changes
-   * Add function mbedtls_ssl_conf_datagram_packing() to configure
+   * Add function mbedtls_ssl_set_datagram_packing() to configure
      the use of datagram packing (enabled by default).
 
 Bugfix