Fix the syntax error

src/emqttd.app.src

@@ -5,7 +5,7 @@
     {registered, [emqttd_sup]},
     {applications, [kernel,stdlib,gproc,lager,esockd,mochiweb,lager_syslog,pbkdf2]},
     {env, []},
-    {mod, {emqttd_app, []}}
+    {mod, {emqttd_app, []}},
     {maintainers, ["Feng Lee <feng@emqtt.io>"]},
     {licenses, ["MIT"]},
     {links, [{"Github", "https://github.com/emqtt/emqttd"}]}