Correctly specify crossbeam-channel as feature.

The feature controlling the usage of `crossbeam_channel` is
not `crossbeam`.

This has been incorrect since it was made optional in be6ef8a.

notify-debouncer-full/src/lib.rs

@@ -123,7 +123,7 @@ where
     }
 }
 
-#[cfg(feature = "crossbeam")]
+#[cfg(feature = "crossbeam-channel")]
 impl DebounceEventHandler for crossbeam_channel::Sender<DebounceEventResult> {
     fn handle_event(&mut self, event: DebounceEventResult) {
         let _ = self.send(event);

notify-debouncer-mini/src/lib.rs

@@ -152,7 +152,7 @@ where
     }
 }
 
-#[cfg(feature = "crossbeam")]
+#[cfg(feature = "crossbeam-channel")]
 impl DebounceEventHandler for crossbeam_channel::Sender<DebounceEventResult> {
     fn handle_event(&mut self, event: DebounceEventResult) {
         let _ = self.send(event);