Update link

[mkaz]

Update link to Python String Format Cookbook

Short description: Adds or changes DOCUMENTATION

Longer description, this may help improve our documentation, please be
as verbose as you wish:

Update link to Python String Format Cookbook, domain and location change.
Redirects are in place now but the domain will expire next year.

[tbielawa]

Thanks so much for the contribution! Very proactive.

I still think that guide is excellent. Thanks for publishing it