change fill_percent example in doc to fill_percent=20

Author: Kully

plotly/dashboard_objs/dashboard_objs.py

@@ -434,7 +434,7 @@ def insert(self, box, side='above', box_id=None, fill_percent=50):
         my_dboard.insert(box_1, 'left', 1)
         my_dboard.insert(box_1, 'below', 2)
         my_dboard.insert(box_1, 'right', 3)
-        my_dboard.insert(box_1, 'above', 4, fill_percent=30)
+        my_dboard.insert(box_1, 'above', 4, fill_percent=20)
 
         my_dboard.get_preview()
         ```