mqtt: FIX: update sample code to include MQTT UNSUBSCRIBE message

This patch demonstrates how to use the MQTT UNSUBSCRIBE message.

The publisher application is updated to fix a typo.

Jira: ZEP-623

Change-Id: I2c302c9f9320687fc39f36726394417db89f06d8
Signed-off-by: Flavio Santes <flavio.santes@intel.com>