doc: the typo train

Context

This MR is the current edition of the typo train, that is, a batch of minor contributions to the Tezos documentation.

In this edition:

• the branch name has been renamed to typo-doc (from typo-train) in order to fulfill the CI branch name convention for doc MRs
• ...

Manually testing the MR

Check that the additions are minor and go into the right direction.

Checklist

• Select suitable reviewers using the Reviewers field below.