docs: fix typo in toh-pt6.md

[volshibenik]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

File toh-pt6.md has a broken link href attribute, that leads to localhost:4800 instead of angular.io, which causes an error.

What is the new behavior?

Link works correctly

Does this PR introduce a breaking change?

• Yes
• No

[mary-poppins]

You can preview c4c3620 at https://pr27403-c4c3620.ngbuilds.io/.
You can preview 7567f44 at https://pr27403-7567f44.ngbuilds.io/.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}