Fix braces in panic message in test.

library/core/tests/nonzero.rs

@@ -85,7 +85,7 @@ fn test_match_option_string() {
     let five = "Five".to_string();
     match Some(five) {
         Some(s) => assert_eq!(s, "Five"),
-        None => panic!("unexpected None while matching on Some(String { ... })"),
+        None => panic!("{}", "unexpected None while matching on Some(String { ... })"),
     }
 }