Update config.json

It's worth running changes through `configlet fmt` as well as `configlet lint`. Exercism used to be relaxed about JSON formatting, recently not so much (I've been pulled up on it by quite senior people).

Author: colinleach

config.json

@@ -62,7 +62,9 @@
         "slug": "high-school-sweetheart",
         "name": "high-school-sweetheart",
         "uuid": "df1ff192-e8d7-4124-862d-1b99979aa5b5",
-        "concepts": ["function-composition"],
+        "concepts": [
+          "function-composition"
+        ],
         "prerequisites": [],
         "status": "wip"
       }