tour: add explanation of sync.Mutex

sync.Mutex is needed to solve the last exercise of the tour

Fixes golang/go#13110

Change-Id: I0b10f8c0fc0e3c656377b93bdbf40e38c5cb762b
Reviewed-on: https://go-review.googlesource.com/17403
Reviewed-by: Rob Pike <r@golang.org>
2 files changed