Fix broken SQL link in Scaling AWS exercise (#258)

Fix broken SQL link in Scaling AWS exercise

solutions/system_design/scaling_aws/README.md

@@ -83,7 +83,7 @@ Handy conversion guide:
 
 * **Web server** on EC2
     * Storage for user data
-    * [**MySQL Database**](https://github.com/donnemartin/system-design-primer#sql)
+    * [**MySQL Database**](https://github.com/donnemartin/system-design-primer#relational-database-management-system-rdbms)
 
 Use **Vertical Scaling**: