Customizer: Correct reference to the 'Add new image' button.

props jjeaton.
see #27890, for trunk.

Built from https://develop.svn.wordpress.org/trunk@28170


git-svn-id: http://core.svn.wordpress.org/trunk@28001 1a063a9b-81f0-0310-95a4-ce76da25c4cd