core: Formatting fix in documentation for task::unkillable

src/libcore/task.rs

@@ -528,6 +528,7 @@ fn get_task() -> task {
  *
  * # Example
  *
+ * ~~~
  * task::unkillable {||
  *     // detach / yield / destroy must all be called together
  *     rustrt::rust_port_detach(po);
@@ -535,6 +536,7 @@ fn get_task() -> task {
  *     task::yield();
  *     rustrt::rust_port_destroy(po);
  * }
+ * ~~~
  */
 unsafe fn unkillable(f: fn()) {
     class allow_failure {