docs: add missing colon after [error]

[algomaster99]

PR checklist

• Addresses an existing issue: fixes Missing colon in documentation #4894
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Add missing colon after [error] in docs/develop/testing-rules/index.md.

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

These changes simply maintain the uniformity of the documentation. :)

[algomaster99]

One test is failing. I think the error is test/rules/strict-boolean-expressions/default/test.ts.lint: Failed!. Shall I try to fix it?

[JoshuaKGoldberg]

Thanks! 🙌