add sentence to 404

Author: vincerubinetti

site/src/pages/404.astro

@@ -8,7 +8,10 @@ import Heading from "@/components/Heading";
   <section>
     <Heading level={1}>Page Not Found</Heading>
 
-    <p>The content you're looking for does not exist or has been moved.</p>
+    <p>
+      The content you're looking for does not exist or has been moved. Check
+      that the URL in your address bar is correct.
+    </p>
 
     <div class="row">
       <Button design="plain" to="/loci">