docs: Update Github logo
<a href="https://www.pulumi.com?utm_campaign=pulumi-pulumi-github-repo&utm_source=github.com&utm_medium=top-logo" title="Pulumi - Modern Infrastructure as Code - AWS Azure Kubernetes Containers Serverless">
- <img src="https://www.pulumi.com/images/logo/logo.svg?" width="350">
+ <img src="https://www.pulumi.com/images/logo/logo-on-white-box.svg?" width="350">
</a>
[![Slack](http://www.pulumi.com/images/docs/badges/slack.svg)](https://slack.pulumi.com?utm_campaign=pulumi-pulumi-github-repo&utm_source=github.com&utm_medium=slack-badge)
