fix md table of contents layout

Author: clarklindev

README.md

@@ -13,7 +13,7 @@ completion date: 2021-10-16
 ---
 ## Table of contents
 
-  [1. SQL](https://github.com/swagfinger/sql_postgreSQL/blob/master/SQL.md)
-  [2. database design](https://github.com/swagfinger/sql_postgreSQL/blob/master/database_design.md)
-  [3. postgreSQL](https://github.com/swagfinger/sql_postgreSQL/blob/master/postgreSQL.md)
+  + [1. SQL](https://github.com/swagfinger/sql_postgreSQL/blob/master/SQL.md)
+  + [2. database design](https://github.com/swagfinger/sql_postgreSQL/blob/master/database_design.md)
+  + [3. postgreSQL](https://github.com/swagfinger/sql_postgreSQL/blob/master/postgreSQL.md)