docs: Fix MQTT code snipet (#1900)

Change-Id: I9e6c98f7ff73e1d5d50d2308a2db773abad0ece1
IoT.js-DCO-1.0-Signed-off-by: Philippe Coval p.coval@samsung.com

Author: rzr

docs/api/IoT.js-API-MQTT.md

@@ -89,7 +89,7 @@ var opts = {
   clientId: 'IoT.js Client',
 }
 
-var subscribe_opts {
+var subscribe_opts = {
   retain: false,
   qos: 2
 }