Syntax highlight TooltipManager documentation

[50Wliu]

Requirements for Contributing Documentation

• Fill out the template below. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• The pull request must only contribute documentation (for example, markdown files or API docs). To contribute other changes, you must use a different template. You can see all templates at https://github.com/atom/atom/tree/master/.github/PULL_REQUEST_TEMPLATE.

Description of the Change

javascript isn't recognized by the docs parser, leading to no syntax highlighting. Use js instead.

Release Notes

N/A