Update d_string example.

examples/d_string.dm

@@ -1,4 +1,16 @@
+class World(pub name: String, pub color: String)
+    pub static worlds = [
+        World('Mercury', 'Red'),
+        World('Earth', 'Blue'),
+        World('Jupiter', 'Orange')
+    ]
+
+    pub static getWorld(color: String) -> String = worlds
+        .find { it.color == color }
+        .map { it.name }
+        .expect "No world has the given color ${color}."
+end
+
 fn main()
-    let world = 'Earth'
-    println "Hello, ${world}!"
-end
+    println "Hello, ${World::getWorld('Blue')}!"
+end