Ensure trailing slash for customize-preview-js src

On a multisite subdirectory install, for a site 'foo', a URL like the following results in a `NOBLOGREDIRECT`:

http://example.com/foo?styles-action=customize-preview-js...

This results in the preview not working at all. The fix is to ensure that the script path ends in a slash, and this helps WordPress Multisite properly route the request to the current site:

http://example.com/foo/?styles-action=customize-preview-js...