from_micros: add feature name

Author: gagath

src/libstd/time/duration.rs

@@ -123,14 +123,15 @@ impl Duration {
     /// # Examples
     ///
     /// ```
+    /// #![feature(duration_from_micros)]
     /// use std::time::Duration;
     ///
     /// let duration = Duration::from_micros(1_000_002);
     ///
     /// assert_eq!(1, duration.as_secs());
     /// assert_eq!(2000, duration.subsec_nanos());
     /// ```
-    #[unstable(feature = "", issue = "44400")]
+    #[unstable(feature = "duration_from_micros", issue = "44400")]
     #[inline]
     pub fn from_micros(micros: u64) -> Duration {
         let secs = micros / MICROS_PER_SEC;