Change class for editor PSM closing transparent div

No issue

- Changes the class used by the transparent div used to close the PSM

Without this change, when in the editor and the nav is open, the transparent div here is above the div the nav is looking for. So, the div for the PSM is clicked, rather than the nav div.
Merge pull request #4014 from PaulAdamDavis/fix-mobile-nav-click

Change class for editor PSM closing transparent div