Fix changelog for ssl_server2.c usage fix

diff --git a/ChangeLog b/ChangeLog
index ad773c7..a89f2a4 100644
--- a/ChangeLog
+++ b/ChangeLog

@@ -10,6 +10,7 @@
      as recommended in RFC 6347 Section 4.1.2.7.
    * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.
      Found by projectgus and jethrogb, #836.
+   * Fix usage help in ssl_server2 example. Found and fixed by Bei Lin.
 
 = mbed TLS 2.6.0 branch released 2017-08-10