Result::unwrap_infallible: change doctest

Changed the example to code that compiles now.

Author: mzabaluev

src/libcore/result.rs

@@ -1079,8 +1079,15 @@ impl<T, E: Into<!>> Result<T, E> {
     /// Basic usage:
     ///
     /// ```
-    /// let x = u64::try_from(42u32).unwrap_infallible();
-    /// assert_eq!(x, 42u64);
+    /// # #![feature(never_type)]
+    /// # #![feature(unwrap_infallible)]
+    /// 
+    /// fn only_good_news() -> Result<String, !> {
+    ///     Ok("this is fine".into())
+    /// }
+    ///
+    /// let s: String = only_good_news().unwrap_infallible();
+    /// println!("{}", s);
     /// ```
     #[inline]
     pub fn unwrap_infallible(self) -> T {