clarify copilot-setup-steps.yml is required in default branch (not main) (#40504)

Co-authored-by: Sharra-writes <sharra-writes@github.com>

Author: jancbeck

content/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment.md

@@ -41,7 +41,7 @@ Instead, you can preconfigure {% data variables.product.prodname_copilot_short %
 A `copilot-setup-steps.yml` file looks like a normal {% data variables.product.prodname_actions %} workflow file, but must contain a single `copilot-setup-steps` job. This job will be executed in {% data variables.product.prodname_actions %} before {% data variables.product.prodname_copilot_short %} starts working. For more information on {% data variables.product.prodname_actions %} workflow files, see [AUTOTITLE](/actions/using-workflows/workflow-syntax-for-github-actions).
 
 > [!NOTE]
-> The `copilot-setup-steps.yml` workflow won't trigger unless it's present on your main branch.
+> The `copilot-setup-steps.yml` workflow won't trigger unless it's present on your default branch.
 
 Here is a simple example of a `copilot-setup-steps.yml` file for a TypeScript project that clones the project, installs Node.js and downloads and caches the project's dependencies. You should customize this to fit your own project's language(s) and dependencies: