Equations (20), (25) and (27) of the previously published article are corrected in the associated PDF file. A new expression for J(U2) is plotted in a corrected version of the previous figure 1. The previous statement that 'extremelyprecise measurements are required for quantitative comparisons in theplateau regime' might be relaxed. The authors are grateful to Matt Landreman for bringing the error inthe paper to their attention.