Fix some broken doc links (#413)

tokio-timer/src/clock/clock.rs

@@ -26,7 +26,7 @@ thread_local!(static CLOCK: Cell<Option<*const Clock>> = Cell::new(None));
 /// execution context. By default, this is `Instant::now()`.
 ///
 /// Note that, because the source of time is configurable, it is possible to
-/// observe non-monotonic behavior when calling [`now`] from different
+/// observe non-monotonic behavior when calling `now` from different
 /// executors.
 ///
 /// See [module](index.html) level documentation for more details.