Ts file snippet is not rendering

**Steps to reproduce the behavior:**
1. Ember install ember-cli-typescript --save
2. Create a route `docs/button`
3. In `docs/button/controller.ts`

```
// BEGIN-SNIPPET docs-button.ts
@action
onClick(){
}
// END-SNIPPET
```
4. In `docs/button/template.hbs`
`{{demo.snippet name='docs-button.ts'}}`

**Version details**
```
"ember-cli": "~3.8.1",
"ember-source": "~3.8.0",
"ember-cli-addon-docs": "^0.6.7",
"ember-cli-typescript": "^2.0.0",
```