etl: downgrade to easy difficulty (#2067)

forum discussion:
https://forum.exercism.org/t/rust-etl-exercise-is-a-bit-odd/16921

Author: senekor

config.json

@@ -679,7 +679,7 @@
         "uuid": "0c8eeef7-4bab-4cf9-9047-c208b5618312",
         "practices": [],
         "prerequisites": [],
-        "difficulty": 4,
+        "difficulty": 1,
         "topics": [
           "btree"
         ]