Fix choropleth example (#688)

Fixes #687 by switching away from wrongly implemented nullish coalescing operator. 

There are probably a couple different ways to do this, so this is just one that seems simple and effective.

Author: garikAsplund

packages/layerchart/src/routes/docs/examples/Choropleth/+page.svelte

@@ -114,7 +114,7 @@
           />
           <Tooltip.Item
             label="Est. Percent under 18"
-            value={d?.percentUnder18 ?? 0 / 100}
+            value={d ? d.percentUnder18 / 100 : 0}
             format="percentRound"
             valueAlign="right"
           />