chore: use explicit main branch for SAM docs

[hoffa]

Issue #, if available

Schema automation was failing; doc_source was deleted from the default branch: https://github.com/awsdocs/aws-sam-developer-guide

Using explicit main until we switch to an alternative source.

Description of changes

Description of how you validated changes

make schema-all

Checklist

• Adheres to the development guidelines
• Add/update transform tests
  • Using correct values
  • Using wrong values
• Add/update integration tests

Examples?

Please reach out in the comments if you want to add an example. Examples will be
added to sam init through aws/aws-sam-cli-app-templates.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[GavinZZ]

Curious on why do we need to specify the branch name? What’s the default cloned branch?

[hoffa]

Curious on why do we need to specify the branch name? What’s the default cloned branch?

Curious on why do we need to specify the branch name? What’s the default cloned branch?

Check the summary/repo; changed to archived.