Fix Relational database management system (RDBMS) link

Author: donnemartin

README.md

@@ -120,14 +120,13 @@ Review the [Contributing Guidelines](https://github.com/donnemartin/system-desig
     * [Microservices](#microservices)
     * [Service discovery](#service-discovery)
 * [Database](#database)
-    * [SQL](#sql)
-        * [Relational database management system (RDBMS)](relational-database-management-system-rdbms)
-        * [Scaling SQL](#scaling-sql)
-            * [Master-slave replication](#master-slave-replication)
-            * [Federation](#federation)
-            * [Sharding](#sharding)
-            * [Denormalization](#denormalization)
-            * [SQL tuning](#sql-tuning)
+    * [Relational database management system (RDBMS)](#relational-database-management-system-rdbms)
+    * [Scaling SQL](#scaling-sql)
+        * [Master-slave replication](#master-slave-replication)
+        * [Federation](#federation)
+        * [Sharding](#sharding)
+        * [Denormalization](#denormalization)
+        * [SQL tuning](#sql-tuning)
     * [NoSQL](#nosql)
         * [Key-value store](#key-value-store)
         * [Document store](#document-store)