Update for example_4_keys_instances for Python

[mattmz]

While running through the Python examples, I noticed a few updates to example 4 that seem to be more inline with the C++ step-by-step flow at https://community.rti.com/static/documentation/connext-dds/6.0.1/doc/manuals/connext_dds/getting_started/cpp11/intro_keys_instances.html#example-chocolate-factory.

Example edits in #50

There may be some confusion as of step 4.5 when the user starts to learn about ChocolateLotState instance disposal. It looked like monitor_lot_state was being called for temperature topics (which aren't explored until 4.6+), leading to misleading output (e.g., "Received lot update: ").

The example changes above seem to fix this, in that the readers/monitors are hooked up correctly, the output matches the guide and they can see the instance disposal in step 4.5.

The user can still later explore the ChocolateTemperature reader support in steps 4.6+.

[mattmz]

Just wanted to see if the above suggestion (#50) is useful for the Python tutorial flow. Thank you!

[mattmz]

@rosemwahlin, Thought this would be a good way to clarify example 4. Thank you!