  There was a slight error in calculating the nonlinear sigma-model prediction
for the 2-magnon scattering continuum shown in Fig. 2 of cond-mat/0010049, and
Phys. Rev. Lett. {\bf 87}, 017202 (2001). Present erratum explains what was
incorrect, and shows a corrected version of the figure. The correction is minor
and does not affect main conclusions drawn in the original paper.
