Update version header format in what's new document

Author: serhiipylypchuk1991

docs/news/whats-new.md

@@ -8,7 +8,7 @@ description: You can explore what's new in DHTMLX Pivot and its release history
 
 If you are updating Pivot from an older version, check [Migration to newer versions](news/migration.md) for details.
 
-# Version 2.1.1
+## Version 2.1.1
 
 Released on June 10, 2026