Fix typos in markdown files

[cclauss]

Description

Please describe what changes you made, and why you feel they are necessary. Make sure to include
code examples, where applicable.

Fix typos in markdown files.
% codespell --ignore-words-list=hda,revered --skip=www/content/examples/active-search.md **/*.md

• https://pypi.org/project/codespell

Corresponding issue:

Testing

Please explain how you tested this change manually, and, if applicable, what new tests you added. If
you're making a change to just the website, you can omit this section.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded