Do not understand nested subgraph

[phuongnd08]

Tried this today, rendered perfectly by https://mermaidjs.github.io/mermaid-live-editor/ yet showing error with the extension:

Code:

```mermaid
graph TD
subgraph "groupA"
  subgraph "groupA1"
    balancer[Balancer] --> backend[Backend]
    balancer --> frontend[Frontend]
  end
end

```

Render: