README.md: Update year

Change-Id: I746fb94d112c7d797e206bca0fd1e13fcd26bae3

README.md

 # The Asterisk(R) Open Source PBX
 ```text
         By Mark Spencer <markster@digium.com> and the Asterisk.org developer community.
-        Copyright (C) 2001-2009 Digium, Inc. and other copyright holders.
+        Copyright (C) 2001-2019 Digium, Inc. and other copyright holders.
 ```
 ## SECURITY