redirect /python/annotated_heatmap/ to /python/annotated-heatmap/

[chriddyp]

Words separated by underscores are roughly treated line one word whereas words separated by hyphens are treated as two words

https://support.google.com/webmasters/answer/76329?hl=en

Consider using punctuation in your URLs. The URL http://www.example.com/green-dress.html is much more useful to us than http://www.example.com/greendress.html. We recommend that you use hyphens (-) instead of underscores (_) in your URLs.

[nicolaskruchten]

Done in plotly/plotly.py@8d4163d and deployed.