Taking "preview" off from "Use a managed identity"

Managed identity is no longer preview, and "preview" needs to be taken off the table of contents.

Author: tysonfms

articles/container-instances/TOC.yml

@@ -103,7 +103,7 @@
       href: container-instances-exec.md
     - name: Run a setup container (init)
       href: container-instances-init-container.md
-    - name: Use a managed identity (preview)
+    - name: Use a managed identity
       href: container-instances-managed-identity.md
     - name: Use GPU resources (preview)
       href: container-instances-gpu.md