ETL: fix indent

3 spaces -> 4 spaces

exercises/etl/src/main/java/Etl.java

 import java.util.Map;
 
 public class Etl {
-   public Map<String, Integer> transform(Map<Integer, List<String>> old) {
-      return null;
-   }
+    public Map<String, Integer> transform(Map<Integer, List<String>> old) {
+        return null;
+    }
 }