Mavlink Tunnel Example, .join called on function not thread

In master branch, the join method is called on the function and not the thread.
```py
tcp_server_loop_thread = threading.Thread(target=tcp_server_loop)
tcp_server_loop_thread.start()

# Running the node until the application is terminated or until first error.
try:
    node.spin()
except KeyboardInterrupt:
    tcp_server_loop.join()  
    if connection:
        connection.close()
```