Add missing proxy property to docstring

[Gr3at]

Closes issues #461.

It's been quite some time since the issue was created and things might have changed.
While attempting to replicate the silent failure (mentioned by the issue creator), I received an informative error message from the PySocks package (socks.ProxyConnectionError: Error connecting to HTTP proxy example-proxy.com:8080: timed out).

To get the above error I just extended the basic publish example to include the proxy_args dictionary, including only the required keys.

# examples/publish_single.py
import context  # Ensures paho is in PYTHONPATH

import paho.mqtt.publish as publish
import socks

proxy_dict={
    "proxy_type": socks.HTTP,
    "proxy_addr": "example-proxy.com",
}

publish.single("paho/test/single", "boo", hostname="mqtt.eclipseprojects.io", proxy_args=proxy_dict)

The fact that I received an error for port 8080 raised a flag of a default value, which is the case.
PySocks has a DEFAULT_PORTS dictionary.

I extended the docstring, adding the port as optional.

[petersilva]

can't accept a contribution with ECA...

[PierreF]

For your contribution to be accepted, you will need to sign the ECA with the email used to do the commit: https://accounts.eclipse.org/user/eca

[Gr3at]

For your contribution to be accepted, you will need to sign the ECA with the email used to do the commit: https://accounts.eclipse.org/user/eca

happy new year.
signed the ECA.
also made a new commit to actually commit with a reply capable email (same as in the ECA)

[PierreF]

Thank for your contribution.