Add doctest

Author: mousetail

library/core/src/fmt/mod.rs

@@ -1827,6 +1827,7 @@ impl<'a> Formatter<'a> {
     /// }
     ///
     /// assert_eq!(&format!("{:+}", Foo(23)), "Foo(+23)");
+    /// assert_eq!(&format!("{:+}", Foo(-32)), "Foo(-23)");
     /// assert_eq!(&format!("{}", Foo(23)), "Foo(23)");
     /// ```
     #[must_use]