Update index.d.ts

Datatables api should be array-like, and accessable with index like array

types/datatables.net/index.d.ts

@@ -28,6 +28,10 @@ declare namespace DataTables {
     }
 
     interface Api extends CoreMethods {
+        /**
+         * API should be array-like
+         */
+        [key: number]: any;
         /**
          * Returns DataTables API instance
          *