Fix simple typo: singletone -> singleton

[timgates42]

Issue Type

[x] Bug (Typo)

Steps to Replicate

1. Examine singleton.py.
2. Search for singletone.

Expected Behaviour

1. Should read singleton.

Semi-automated issue generated by
https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

To avoid wasting CI processing resources a branch with the fix has been
prepared but a pull request has not yet been created. A pull request fixing
the issue can be prepared from the link below, feel free to create it or
request @timgates42 create the PR. Alternatively if the fix is undesired please
close the issue with a small comment about the reasoning.

https://github.com/timgates42/Design-Patterns-in-Python/pull/new/bugfix_typo_singleton

Thanks.