[JENKINS-75311] Add closing quote in checkout scmGit branches example

[ruyut]

[JENKINS-75311] Fix missing closing quote in checkout scmGit branches argument

Fixed the incorrect checkout scmGit syntax, changing branches: [[name: 'main]] to branches: [[name: 'main']].

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[MarkEWaite]

Thanks very much! Nice detection of a bug in the example. The bug was first introduced in 19dcec3 when I updated an example.