Update sql-database-elastic-scale-manage-credentials.md

This is from stuarto@microsoft.com, GPM on the product.   We have had several customer inquiries related to the format of User IDs in connection strings.
Update sql-database-elastic-scale-manage-credentials.md

From Stuart Ozer -- my corrections on initial set of changes
Update sql-database-elastic-scale-manage-credentials.md
Merge pull request #2265 from stuartozer/patch-1

Updates to Elastic Scale Credentials topic from stuarto