demo: add data sources to nextzen tile attribution

Author: bcamper

demos/scene.yaml

@@ -103,7 +103,10 @@ sources:
         # zooms: [0, 2, 4, 6, 8, 10, 12, 14, 16] # only load tiles every 2 zooms (overrides max_zoom)
         url_params:
             api_key: global.api_key
-        attribution: Tiles by <a href="https://www.nextzen.org/" target="_blank">Nextzen</a>
+        attribution: |
+            Tiles by <a href="https://www.nextzen.org/" target="_blank">Nextzen</a>
+            w/data from <a href="https://www.openstreetmap.org/" target="_blank">OpenStreetMap</a> and
+            <a href="https://whosonfirst.org/" target="_blank">Who's On First</a>
         # request_headers: # send custom headers with tile requests
         #     Authorization: Bearer xxxxxxxxx
         # Data filtering demo with 'scripts' and 'transform' properties: