Use modern syntax in git step online help

[MarkEWaite]

Use modern syntax in git step online help

The "$class: 'GitSCM'" syntax has not been needed for many releases.

Found while reviewing the online help for the git step in the Pipeline steps reference.

Checklist

• I have read the CONTRIBUTING doc
• I have referenced the Jira issue related to my changes in one or more commit messages
• I have added tests that verify my changes
• Unit tests pass locally with my changes
• I have added documentation as necessary
• No Javadoc warnings were introduced with my changes
• No spotbugs warnings were introduced with my changes
• Documentation in README has been updated as necessary
• Online help has been added and reviewed for any new or modified fields
• I have interactively tested my changes
• Any dependent changes have been merged and published in upstream modules (like git-client-plugin)

Types of changes

What types of changes does your code introduce?

• Documentation