Update $unset href to correct path

Author: abinav2307

articles/cosmos-db/mongodb/vcore/operators/field-update/toc.yml

@@ -15,4 +15,4 @@
 - name: $setOnInsert
   href: $setoninsert.md
 - name: $unset
-  href: $unset.md
+  href: ../aggregation/$unset.md