Fix Azure Container Apps identity assign CLI flag (#51740)

Change --identities to --user-assigned for the az containerapp identity
assign command to match the correct Azure CLI syntax.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: scottaddie

docs/azure/sdk/authentication/user-assigned-managed-identity.md

@@ -110,7 +110,7 @@ The Azure CLI provides different commands to assign a user-assigned managed iden
     az containerapp identity assign \
         --resource-group <resource-group-name> \
         --name <containerapp-name> \
-        --identities <user-assigned-identity-resource-id>
+        --user-assigned <user-assigned-identity-resource-id>
     ```
 
     For Azure Virtual Machines, use the Azure CLI command [`az vm identity assign`](/cli/azure/vm/identity#az-vm-identity-assign):