Update default branch from master to main

Hi Buildkite :wave:

I'm going through the getting started guide and created my first agent with the bash example. I encountered a hurdle when running my first build. The pipeline defaults expect `main` as the default branch but only `master` exists on this repository.

I think two things can be done here to get your documentation in sync with the example repositories:

1. We could update repository's default to `main`
2. We could tweak the [getting started documentation](https://buildkite.com/docs/tutorials/getting-started#install-and-run-your-first-agent) a little bit on point number 5 of the section `Add the sample pipeline` (see screenshot below)

## 1. Default branch: main

My guess is that you now default all repository branches to `main` when creating pipelines (which is :+1:) but this repository still has `master` as the default and new users trying out agents don't necessarily know that. The other getting started repository [powershell-example](https://github.com/buildkite/powershell-example) has `main` as its default branch so all good there.

Here are screenshots of the issue:

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## 2. Documentation update

The documentation seems to suggest that master is still the default for repositories and conflicts with Buildkite pipeline defaults. The doc could be updated to:

> You can usually accept the defaults here. If your repository uses 'master' rather than 'main' branch, edit the Branch option. Select Create build. Buildkite takes you to the pipeline build.

### Current documentation