Update output of UI test to capture improved error message. (#35)

tests/ui/must_use.stderr

@@ -1,14 +1,22 @@
-error: unused return value of `apples` that must be used
+error: unused pinned boxed `Future` trait object that must be used
  --> tests/ui/must_use.rs:9:5
   |
 9 |     apples(3);
   |     ^^^^^^^^^
   |
+  = note: futures do nothing unless you `.await` or poll them
 note: the lint level is defined here
  --> tests/ui/must_use.rs:1:9
   |
 1 | #![deny(unused_must_use)]
   |         ^^^^^^^^^^^^^^^
+
+error: unused return value of `apples` that must be used
+ --> tests/ui/must_use.rs:9:5
+  |
+9 |     apples(3);
+  |     ^^^^^^^^^
+  |
 help: use `let _ = ...` to ignore the resulting value
   |
 9 |     let _ = apples(3);