adding article that was missing from toc

Author: julieMSFT

articles/synapse-analytics/sql-data-warehouse/toc.yml

@@ -8,7 +8,8 @@
     href: sql-data-warehouse-overview-what-is.md
   - name: Azure Synapse Analytics architecture
     href: massively-parallel-processing-mpp-architecture.md
-
+  - name: Data warehouse units
+    href: what-is-a-data-warehouse-unit-dwu-cdwu.md
   - name: Cheat sheet
     href: cheat-sheet.md
   - name: Best practices