docs: fix field name typo for interactive_area_brush example

[dsmedia]

PR Description

The interactive_area_brush entry in examples.json has "descripton" (missing an i) instead of "description". Because create-example-pages checks example.description, the misspelled field is silently ignored, and the gallery page renders with no description text.

The entry has a description, but it never renders because of the typo:

In this example, we apply an interval selection to select subset of data in an area chart. The selected data is highlighted in gold by the second layer of an area mark that filters its data by the brush selection.

The gallery page currently shows no description text:

Just for future reference, create-example-pages first checks for a description field in the example's examples.json entry. If none is found, it falls back to the description field in the .vl.json spec file. In this case, examples.json has the description under a misspelled key, and the spec file has no description at all — so neither source provides one.

Found while working on gallery example metadata in vega/vega-datasets#776.

Checklist

• This PR is atomic (i.e., it fixes one issue at a time).
• The title is a concise semantic commit message (e.g. "fix: correctly handle undefined properties").
• npm test runs successfully
• For new features:
  • Has unit tests.
  • Has documentation under site/docs/ + examples.

Tips:

• https://medium.com/@greenberg/writing-pull-requests-your-coworkers-might-enjoy-reading-9d0307e93da3 is a nice article about writing a nice PR.
• Use draft PR for work in progress PRs / when you want early feedback (https://github.blog/2019-02-14-introducing-draft-pull-requests/).

[domoritz]

Thanks! Next time, you can make a branch in this fork so that the CI can run fully (the format one).