Duplicated link in parallel-letter-frequency documentation.

[adamazing]

Spotted a wee issue in the parallel-letter-frequency documentation:

- [Concurrency in the Golang Book](https://www.golang-book.com/books/intro/10)  <----+
- [A Tour of Go's concurrency section](https://tour.golang.org/concurrency/1)        |-- same page club
- [Golang Book: Concurrency](https://www.golang-book.com/books/intro/10)        <----+

See: parallel-letter-frequency/.docs/instructions.append.md#L31-L33

[github-actions]

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