tests: update needless_bool test stderr

I'm getting a test failure otherwise with rust master (f688ba6).

Author: matthiaskrgr

tests/ui/needless_bool.stderr

@@ -145,7 +145,7 @@ LL | |         false
 LL | |     } else {
 LL | |         true
 LL | |     };
-   | |_____^ help: you can reduce it to: `{ !returns_bool() }`
+   | |_____^ help: you can reduce it to: `!returns_bool()`
 
 error: aborting due to 16 previous errors