App name is not in valid format

```
File "/Users/ngjhn/Library/Python/3.7/lib/python/site-packages/jsii/_kernel/__init__.py", line 282, in create
for iface in getattr(klass, "__jsii_ifaces__", [])
File "/Users/ngjhn/Library/Python/3.7/lib/python/site-packages/jsii/_kernel/providers/process.py", line 344, in create
return self._process.send(request, CreateResponse)
File "/Users/ngjhn/Library/Python/3.7/lib/python/site-packages/jsii/_kernel/providers/process.py", line 326, in send
raise JSIIError(resp.error) from JavaScriptError(resp.stack)
jsii.errors.JSIIError: Stack name must match the regular expression: /^[A-Za-z][A-Za-z0-9-]*$/, got 'lambda_app'
Subprocess exited with error 1
```

In app.py the name should be `lambda-app` not `lambda_app`