Fix dead link in 3_10_threads

[Hjklvfr]

Replace link https://stjepang.github.io/2017/08/13/designing-a-channel.html to
https://github.com/eshikafe/stjepang.github.io/blob/master/_posts/2017-08-13-designing-a-channel.md

[tyranron]

@Hjklvfr thanks!