updates the toc file

Author: varun-dhawan

articles/postgresql/TOC.yml

@@ -5,7 +5,7 @@
   - name: What is Azure DB for PostgreSQL?
     href: single-server/overview.md
   - name: Which PostgreSQL deployment option to choose?
-    href: single-server/overview-postgres-choose-server-options.md
+    href: flexible-server/overview-postgres-choose-server-options.md
   - name: Service updates
     items:
     - name: SSL root certificate expiring