Fix broken link

[ollipal]

In the concurrency chapter there is a broken link, which is accessible through Wayback machine.

It seems that the author moved it from dustin.github.com to dustin.sallings.org. This commit fixes the link for all translations.

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @ollipal to sign the Salesforce Inc. Contributor License Agreement.