In template `Source Code` tab, make `main.tf` the default

Version: [`v0.20.0+00860cf`](https://github.com/coder/coder/commit/00860cf1c8116e76448f0128abd5364ca100e293)

The README.md is so inconsequential, most people want to see the Terraform and not markdown.

![image](https://user-images.githubusercontent.com/2022166/226906290-25e36148-fb86-48d5-bf7d-5fbbe288281e.png)
