Incorrect Code Example for Ping Demo in Documentation

[kaneki003]

The code example for the Ping Demo on py-libp2p ReadTheDocs has been inadvertently replaced by the Echo Demo.

Suggested Fix:

In the file ./docs/examples.ping.rst, update the current code snippet:

.. literalinclude:: ../examples/echo/echo.py
:language: python
:linenos:

to the correct snippet:

.. literalinclude:: ../examples/ping/ping.py
:language: python
:linenos:

This will ensure the documentation accurately reflects the Ping Demo code.