Doc edit: GeoJSON format used in Plotly follows the 2008 specification

[Tortar]

Description

I used Mapshaper to modify the GeoJSON I was working on and then I had an hard time (like others) understanding that the format of the GeoJSON has to follow the 2008 specification to be understood by Plotly. I updated the docs so that it's specified that Plotly needs this format.

Documentation PR

• [x] I've seen the doc/README.md file
• [x] This change runs in the current version of Plotly on PyPI and targets the doc-prod branch OR it targets the master branch