fix typo

Author: newpavlov

src/libcore/time.rs

@@ -639,7 +639,7 @@ impl Duration {
     ///
     /// let dur = Duration::new(2, 700_000_000);
     /// // note that due to rounding errors result is slightly different
-    /// // from 8.478 anf 847800.0
+    /// // from 8.478 and 847800.0
     /// assert_eq!(dur.mul_f32(3.14), Duration::new(8, 478_000_640));
     /// assert_eq!(dur.mul_f32(3.14e5), Duration::new(847799, 969_120_256));
     /// ```