minor toc string changes to Lcase some entries

Author: craigg-msft

docs/database-engine/TOC.md

@@ -1,9 +1,9 @@
 # Overview
-## [SQL Server Database Engine Backward Compatibility](sql-server-database-engine-backward-compatibility.md)  
-## [Deprecated Database Engine Features in SQL Server 2016](deprecated-database-engine-features-in-sql-server-2016.md)  
-## [Discontinued Database Engine Functionality in SQL Server 2016](discontinued-database-engine-functionality-in-sql-server-2016.md)  
-## [Breaking Changes to Database Engine Features in SQL Server 2017](breaking-changes-to-database-engine-features-in-sql-server-2017.md)  
-## [Breaking Changes to Database Engine Features in SQL Server 2016](breaking-changes-to-database-engine-features-in-sql-server-2016.md)  
+## [SQL Server Database Engine backward compatibility](sql-server-database-engine-backward-compatibility.md)  
+## [Deprecated Database Engine features in SQL Server 2016](deprecated-database-engine-features-in-sql-server-2016.md)  
+## [Discontinued Database Engine functionality in SQL Server 2016](discontinued-database-engine-functionality-in-sql-server-2016.md)  
+## [Breaking changes to Database Engine features in SQL Server 2017](breaking-changes-to-database-engine-features-in-sql-server-2017.md)  
+## [Breaking changes to Database Engine features in SQL Server 2016](breaking-changes-to-database-engine-features-in-sql-server-2016.md)  
 
 # [Install](install-windows/index.md)
 # [Availability groups](availability-groups/windows/index.md)