Make repeat example more expressive

This commit adds the zero-based index of current iteration from the passed HOF "action" of the repeat function to its associated code sample.

KT-20357

libraries/stdlib/samples/test/samples/misc/controlFlow.kt

@@ -15,9 +15,14 @@ class ControlFlow {
         repeat(3) {
             println("Hello")
         }
+        
+        // greets with an index
+        repeat(3) { index ->
+            println("Hello with index $index")
+        }
 
         repeat(0) {
             error("We should not get here!")
         }
     }
-}
+}