On page 347, in Eq. (20) the expression:
$$\displaystyle{Q \over {D^2}} = \displaystyle{{({\rm \Gamma} - 1)} \over {2{({\rm \Gamma} + 1)}^2}}$$
should be replaced by:
$$\displaystyle{Q \over {D^2}} = \displaystyle{1 \over {2({\rm \Gamma} ^2 - 1)}}.$$
All other calculations in the paper are based on the correct expression provided in this Corrigendum.
