Updated commands to properly install git plugin

The git plugin depends on the scm plugin, which must be in the jenkins.war in the correct order.
There's no "order" in a zip file, I don't get this comment.

Please note the preferred way to create a custom jenkins image with your own set of plugins is to create a derived Dockerfile. There's no plan to have additional plugins in the Docker official image, if you think git plugin should be included in default jenkins.war then this should be discussed on jenkins-ci dev list.