Add trait bound to example for MutexGuard that is !Send + Sync (#1358)

This example only makes sense (and is therefore easier to understand
logically :) ) if T is Sync. See
rust-lang/rust#41622 - this used to be a bug
initially.

Author: fbornhofen

src/concurrency/send-sync/examples.md

@@ -28,7 +28,7 @@ Typically because of interior mutability:
 
 These types are thread-safe, but they cannot be moved to another thread:
 
-* `MutexGuard<T>`: Uses OS level primitives which must be deallocated on the
+* `MutexGuard<T: Sync>`: Uses OS level primitives which must be deallocated on the
   thread which created them.
 
 ## `!Send + !Sync`