Python code snippet for caching is incorrect

https://docs.couchbase.com/python-sdk/4.0/howtos/caching-example.html

The Cache Miss code snippet seems to be incorrect as it is not showing the code example on how to fetch from persistent storage.

The .NET SDK does this correctly, for reference.

https://docs.couchbase.com/dotnet-sdk/current/howtos/caching-example.html

Acceptance Criteria:

• Fix the python code example to show what user should do when cache miss happens.