documentation: rename extra_highlight_themes to extra_syntaxes_and_themes

[luishfonseca]

extra_highlight_themes does not exist, this is a typo.
The config option that exists and should be used is called extra_syntaxes_and_themes

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

(Delete or ignore this section for documentation changes)

• Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• Have you created/updated the relevant documentation page(s)?

[Keats]

Thanks!