Rollup merge of #23048 - davbo:fix-broken-link-in-old-guide, r=brson

 Having come back to rust recently after > 6months I was looking for docs
on tasks and stumbled upon this broken link.

Author: Manishearth

src/doc/guide-tasks.md

@@ -1,4 +1,4 @@
 % The (old) Rust Threads and Communication Guide
 
 This content has moved into
-[the Rust Programming Language book](book/tasks.html).
+[the Rust Programming Language book](book/concurrency.html).