Can't be deployed without special configuration

I tried to deploy with the readme configuration and got a CloudFormation error.

```
CREATE_FAILED AWS::CodePipeline::Webhook Webhook Action with name Source was not found in the first stage of the pipeline
```

The cause seems to be the name of the webhook target_action.

https://github.com/boltops-tools/pipedream/blob/1d9e2ebb971448df58428512cebc391138d1124d/lib/pipedream/webhook.rb#L41

I have avoided this problem by setting up the following.

```pipeline.rb
stage "Source" do
github(
source_name: 'Source',
source: "tongueroo/demo-ufo",
auth_token: ssm("/github/user/token")
)
end
stage "DeployStacks" do
codebuild({ name: "demo1, input_artifacts: [name: "SourceArtifact"]})
end
```

It looks like a bug to me, how about it?