Documentation: do not use FlowWidget as base class in examples

[penguinolog]

FlowWidget is deprecated

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

(P. S. If this pull requests fixes an existing issue, please specify which one.)

[coveralls]

coverage: 73.566%. remained the same when pulling 0665838 on penguinolog:examples_deprecated into 7ab8347 on urwid:master.