Adds `default_branch` example to `project_config`
Adds `default_branch` example to `project_config`

The example config makes extensive use of "main" as the default project branch, but the setting isn't present in the example config.
It falls under the umbrella `project_config` (with a full list of settings available via GitLab documentation), but given that it's relevant to other options in the config file, I've added it here as well.
egnyte/gitlabform,egnyte/gitlabform