Update return type for BottleSong stub (#2411)

Author: BNAndras

exercises/practice/bottle-song/BottleSong.cs

@@ -1,7 +1,9 @@
+using System.Collections.Generic;
+
 public static class BottleSong
 {
-    public static string Recite(int startBottles, int takeDown)
+    public static IEnumerable<string> Recite(int startBottles, int takeDown)
     {
         throw new NotImplementedException("You need to implement this method.");
     }
-}
+}