Fix typo in “On Using Keys and Biometrics” page

[sftim]

Fixes #1269 (typo in Markdown docs)

(Required: this reference (one or many) will be closed upon merge. Ideally it has the acceptance criteria and designs for features or fixes related to the work in this Pull Request.)

https://github.com/mozilla-lockwise/lockwise-android/blob/master/docs/architecture/sec-apis.md

Testing and Review Notes

I have confirmed that the revised Markdown does not contain the typo

To Do

• add “WIP” to the PR title if pushing up but not complete nor ready for review
• double check the original issue to confirm it is fully satisfied
• add testing notes and screenshots in PR description to help guide reviewers
• add unit tests
  • optional: consider adding instrumentation (integration/UI) tests
• consider running this branch in a debug simulator and check for memory leak notifications or any warnings
• request the "UX" team perform a design review (if/when applicable)
• make sure CI builds are passing (e.g.: fix lint and other errors)
• check on the accessibility of any added UI