aws_cdk: Stack.format_arn mentions non-existent sep parameter

### Describe the issue

The documentation for the `arn_format` parameter to `Stack.format_arn` currently states:

> The specific ARN format to use for this ARN value. Default: - uses value of sep as the separator for formatting, ArnFormat.SLASH_RESOURCE_NAME if that property was also not provided

However, there is no `sep` property.

### Links

https://docs.aws.amazon.com/cdk/api/v2/python/aws_cdk/Stack.html#aws_cdk.Stack.format_arn

Start at the Stack.format_arn entry point and compare the arn_format parameter documentation with the API page linked in the issue. Remove or correct the reference to the nonexistent sep property, then regenerate or inspect the resulting documentation to confirm the description no longer mentions it.

Written by the indexing model from the issue text.