Update typedefRule.ts

[bumbleblym]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Avoid brittle documentation.

Is there anything you'd like reviewers to focus on?

CHANGELOG.md entry:

[palantirtech]

Thanks for your interest in palantir/tslint, @bumbleblym! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.