Update obsolete argument in @observe decorator.

Author: HaudinFlorence

examples/Heatmap_with_colormap.ipynb

@@ -50,7 +50,7 @@
    "outputs": [],
    "source": [
     "m = ipyleaflet.Map(center=[0, 0], zoom=2)\n",
-    "heat = ipyleaflet.Heatmap(locations=create_random_data(1000), radius=20, blur=10, gradient={0.25 : 'orange', 0.85 : 'red'})\n",
+    "heat = ipyleaflet.Heatmap(locations=create_random_data(1000), radius=20, blur=10, gradient={0.1 : 'orange', 0.2 : 'red', 1.0 :'black'})\n",
     "colormap_control = ipyleaflet.ColormapControl(\n",
     "        caption='Intensity',\n",
     "        colormap=heat.colormap,\n",