style: no punctuation or capitalization in errors

pkg/iterator/iterator.go

@@ -32,7 +32,7 @@ func (i Iterator[T]) Peek() (T, error) {
 	var null T
 
 	if i.IsDone() {
-		return null, fmt.Errorf("Iterator is exhausted.")
+		return null, fmt.Errorf("iterator is exhausted")
 	}
 
 	return i.data[i.index], nil