[FIX] Returns the keys type of the batch fn to ReadonlyArray (#219)

src/index.d.ts

@@ -71,7 +71,7 @@ declare namespace DataLoader {
   // A Function, which when given an Array of keys, returns a Promise of an Array
   // of values or Errors.
   export type BatchLoadFn<K, V> =
-    (keys: ArrayLike<K>) => PromiseLike<ArrayLike<V | Error>>;
+    (keys: ReadonlyArray<K>) => PromiseLike<ArrayLike<V | Error>>;
 
   // Optionally turn off batching or caching or provide a cache key function or a
   // custom cache instance.