Update NuttX-8.1 release date in the ChangeLog.
* apps/examples/chrono: Add chronometer apps example. From Alan Carvalho
de Assis (2019-07-17).
-8.1 2019-09-15 Gregory Nutt <gnutt@nuttx.org>
+8.1 2019-09-14 Gregory Nutt <gnutt@nuttx.org>
* wireless/gs2200m: Add udp support to gs2200m_main.c. From Masayuki
Ishikawa (2019-07-25).
