This example was broken

The provided example to the `sign_plus` method on `fmt` is broken, it displays the `-` sign twice for negative numbers.

Author: mousetail

library/core/src/fmt/mod.rs

@@ -1819,7 +1819,7 @@ impl<'a> Formatter<'a> {
     ///             write!(formatter,
     ///                    "Foo({}{})",
     ///                    if self.0 < 0 { '-' } else { '+' },
-    ///                    self.0)
+    ///                    self.0.abs())
     ///         } else {
     ///             write!(formatter, "Foo({})", self.0)
     ///         }