Auto merge of #28969 - chrisccerami:link_to_ffi_in_concurrency_chapter, r=steveklabnik

Author: bors

src/doc/trpl/concurrency.md

@@ -35,10 +35,12 @@ channel connecting two threads, we would want to be able to send some data
 down the channel and to the other thread. Therefore, we'd ensure that `Send` was
 implemented for that type.
 
-In the opposite way, if we were wrapping a library with FFI that isn't
+In the opposite way, if we were wrapping a library with [FFI][ffi] that isn't
 threadsafe, we wouldn't want to implement `Send`, and so the compiler will help
 us enforce that it can't leave the current thread.
 
+[ffi]: ffi.html
+
 ### `Sync`
 
 The second of these traits is called [`Sync`](../std/marker/trait.Sync.html).