Auto merge of #21152 - steveklabnik:tasks, r=huonw

This moves the "Tasks" chapter to a "Concurrency" one, as it's about threads, but also about how to deal with concurrency issues.

r? @aturon

Author: bors

src/doc/trpl/SUMMARY.md

@@ -27,7 +27,7 @@
     * [Iterators](iterators.md)
     * [Generics](generics.md)
     * [Traits](traits.md)
-    * [Threads](threads.md)
+    * [Concurrency](concurrency.md)
     * [Error Handling](error-handling.md)
     * [Documentation](documentation.md)
 * [III: Advanced Topics](advanced.md)