Typo in Links & Sharing doc

While taking a look at ampstart.com documentation, I have noticed a small typo in the [Links and sharing section](https://ampstart.com/components#links-and-sharing)
The typo is in the class `ampstart-rekated-article-readmore` of the `READ MORE` link
Just `rekated` instead of `related` 🤓

A quick search reveals, the typo in four files:
https://github.com/ampproject/ampstart/search?q=rekated&unscoped_q=rekated

Start by searching the repository for the exact class name `ampstart-rekated-article-readmore`, as the issue reports four occurrences. Correct the spelling in all four files, then verify the Links and sharing section at ampstart.com/components#links-and-sharing and confirm no `rekated` occurrences remain.

Written by the indexing model from the issue text.