Add flatMap sample KT-20357

libraries/stdlib/samples/test/samples/collections/collections.kt

@@ -502,6 +502,12 @@ class Collections {
             assertPrints(numbers.map { it * it }, "[1, 4, 9]")
         }
 
+        @Sample
+        fun flatMap() {
+            val list = listOf("123", "45")
+            assertPrints(list.flatMap { it.toList() }, "[1, 2, 3, 4, 5]")
+        }
+
         @Sample
         fun take() {
             val chars = ('a'..'z').toList()