Herbert Reiter

Add a missing pipe at the end of a mdBook table row.

Appendix 4: Remove duplicate text

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Appendix 4 seems to have duplicate text. This PR deletes the unnecessary part.

Update link URL to section “Returning Errors from the run function” in ch17-03-more-futures.md

Unify rendering for keyboard shortcuts

Removes an unused link in chapter 20-1

The text sometimes still refers to `if let` instead to `let...else`. This PR updates the text accordingly.

Fixes a URL link that refers to localhost with a fixed port number. Removes two unused links.

An open bracket seems to have been lost during a text update. This PR inserts the bracket again, hopefully at the right place.