Builds in readthedocs fail since updates in plot example

[RodolfoFerro]

I've modified the example plot.md to consider a larger database, as detailed in #8.

For some reason, builds in readthedocs are failing: https://readthedocs.org/projects/jupysql/builds/

I created a new branch with diferent table sizes to verify if it was the problem and also tried with a bigger timeout from jupyterbook config. In all cases builds failed, so we need to verify why this is happening and build the new docs with the updated examples.

[RodolfoFerro]

We can run builds from other branches for testing. Once it is solved, it must be changed back to master in readthedocs's config.