fixed the docs

[mattjackson220]

What does this PR (Pull Request) do?

• This PR is not related to any Issue
  tiny fix so the docs build successfully

Which Traffic Control components are affected by this PR?

• Documentation

What is the best way to verify this PR?

make sure docs build

If this is a bug fix, what versions of Traffic Control are affected?

The following criteria are ALL met by this PR

no tests, changelog, or database updates

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[mattjackson220]

fixed in PR #4887