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>

LearnConcurrency.md

diff --git a/LearnConcurrency.md b/LearnConcurrency.md
index e735e59..ec82543 100644
--- a/LearnConcurrency.md
+++ b/LearnConcurrency.md
@@ -7,7 +7,7 @@
 ## Beginner
 
 - Read [Effective Go: Concurrency](https://go.dev/doc/effective_go#concurrency)
-- Watch [Simulating a real-world system in Go](https://www.dotconferences.com/2017/11/sameer-ajmani-simulating-a-real-world-system-in-go)
+- Watch [Simulating a real-world system in Go](https://youtu.be/_YK0viplIl4?si=OW0rXwlypPFEuLh4)
 - Study [The Go Programming Language Specification](https://go.dev/ref/spec), especially
   - [Go statements](https://go.dev/ref/spec#Go_statements)
   - [Channel types](https://go.dev/ref/spec#Channel_types)