Fix custom_widget example

examples/custom_widget.rs

@@ -18,7 +18,7 @@ impl<'a> Default for Label<'a> {
 }
 
 impl<'a> Widget for Label<'a> {
-    fn draw(&self, area: &Rect, buf: &mut Buffer) {
+    fn draw(&mut self, area: &Rect, buf: &mut Buffer) {
         buf.set_string(area.left(), area.top(), self.text, &Style::default());
     }
 }