Capitalize /understanding/{subject} as approprite

[#54951278]

SIDEBAR.md

@@ -31,5 +31,5 @@ Spring Boot provides a wide range of support for various services, including mos
 * [REST][u-rest]
 * [JSON][u-json]
 
-[u-rest]: /understanding/rest
-[u-json]: /understanding/json
+[u-rest]: /understanding/REST
+[u-json]: /understanding/JSON