webadmin: fix incorrect getter in StandardBillingService

module:  webadmin (billing)

The getPopupHeight() returns the width value.

The error is so minor and the patch so new (it has not been merged to 2.6 yet) that the fix requires no release notes.

Patch: http://rb.dcache.org/r/5646/
Target: master
Request: 2.6
Require-book: no
Require-notes: no
Acked-by: Gerd