fix typo in Add Buttons to ModeBar example code

[Lexachoc]

Fix typo in
https://plotly.com/javascript/configuration-options/#add-buttons-to-modebar

The modebardisplay should be put in config instead of layout, but the name is actually wrong. It should be displayModeBar.

And since this example shows how to add a custom button, it is pointless to disable it.

I set it to true so the modebar is always visible, but it is optional.

[LiamConnors]

Thanks for the PR. Sorry for the delay in getting it merged