C43840: Fixing title as per related link title

Translator has reported possible source content issue. 
Description: Unfortunately, the meaning of the source is not quite clear. I fixed it accordingly to the related link title https://github.com/MicrosoftDocs/azure-docs-pr/blob/master/articles/sql-database/sql-database-elastic-pool-failover-group-tutorial.md

Author: v-maudel

articles/sql-database/toc.yml

@@ -514,7 +514,7 @@
       href: sql-database-active-geo-replication-portal.md
     - name: Set up Azure SQL Data Sync
       href: sql-database-get-started-sql-data-sync.md
-    - name: Add to a failover group to Elastic pool
+    - name: Add an Elastic pool to a failover group
       href: sql-database-elastic-pool-failover-group-tutorial.md
     - name: Configure security
       href: sql-database-security-tutorial.md