  The earlier paper [LK] (arXiv:0910.3432) contains a lower bound of the
solution in terms of its $L^1$ norm, which is incorrect. In this note we
explain the mistake and present a correction to it under the restriction that
the permeability constant $m$ satisfies $1< m <2$. As a consequence, the
quantitative estimates on the convergence rate (Main Theorem (c) and Theorem
3.6 in [LK] only hold for $1<m<2$. For $m\geq 2$ a partial convergence rate is
obtained.
