Remove duplicate import in documentation

[fortierq]

It is a very small modification to documentation: no substantial changes.

Please uncomment this block and take a look at this checklist if your PR is making substantial changes to documentation/impacts files in the doc directory. Check all that apply to your PR, and leave the rest unchecked to discuss with your reviewer! Not all boxes must be checked for every PR :)

Documentation PR

• I've seen the doc/README.md file
• This change runs in the current version of Plotly on PyPI and targets the doc-prod branch OR it targets the master branch
• If this PR modifies the first example in a page or adds a new one, it is a px example if at all possible
• Every new/modified example has a descriptive title and motivating sentence or paragraph
• Every new/modified example is independently runnable
• Every new/modified example is optimized for short line count and focuses on the Plotly/visualization-related aspects of the example rather than the computation required to produce the data being visualized
• ...

[nicolaskruchten]

Thanks!