Fix formatting of scmGit Pipeline help

[MarkEWaite]

Fix formatting of scmGit Pipeline help

Was displaying red text for most of the page at

https://www.jenkins.io/doc/pipeline/steps/params/scmgit/

The <code> tag that opened on the checkout keyword was not closed. Close that tag so that the page renders correctly in the Pipeline steps reference on the Jenkins documentation site.

The Pipeline steps reference inside the running Jenkins controller was not affected. It seems to ignore the <code> tags.

Testing done

Code review confirms that the closing and opening tags are matched.

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 demonstrate the feature works or the issue is fixed