LearnConcurrency: fix broken link Change-Id: I044a2023cbbb838f78daba8c9e9ff247ba41aa12 GitHub-Last-Rev: e1b7d6499234ecd4848edd8e6ac995d6ece74f9a GitHub-Pull-Request: golang/wiki#79 Reviewed-on: https://go-review.googlesource.com/c/wiki/+/746340 Reviewed-by: Sean Liao <sean@liao.dev>