Fixed pub_sub link to point to correct example (#2332)

Author: tnyjhnsn

website/versioned_docs/version-0.19.0/concepts/agents.mdx

@@ -68,5 +68,5 @@ UI thread \(i.e. Job or Context\).
 
 ## Further reading
 
-* The [pub\_sub](https://github.com/yewstack/yew/tree/master/examples/pub_sub) example shows how
+* The [pub\_sub](https://github.com/yewstack/yew/tree/yew-v0.19.3/examples/pub_sub) example shows how
 components can use agents to communicate with each other.