TimestampedGeoJson also accepts Polygon, add to docstring (#893)

The `TimestampedGeoJson` class also works when the geometry type is Polygon. Change the docstring to reflect that.

Author: andy23512

folium/plugins/timestamped_geo_json.py

@@ -16,8 +16,8 @@ class TimestampedGeoJson(MacroElement):
     into a map with Map.add_child.
 
     A geo-json is timestamped if:
-    * it contains only features of types LineString, MultiPoint, MultiLineString
-      and MultiPolygon.
+    * it contains only features of types LineString, MultiPoint, MultiLineString,
+      Polygon and MultiPolygon.
     * each feature has a 'times' property with the same length as the
       coordinates array.
     * each element of each 'times' property is a timestamp in ms since epoch,