doc: update simple example with for-loop

doc/sample/Simple.im

@@ -1,3 +1,5 @@
-var y = 32.
-var f = [ :x | x * 2 + y ].
-var result = f(4).
+var x = "Hello, world!".
+
+for (ch, cy) in x do
+    cout put:ch
+end