  We correct a mistake in \cite{St} leading to erroneous formulas in Theorems
5.2 and 5.4. As an immediate corollary of a formula in \cite{BCJ} we give a
formula, which relates the Hecke operators $T(p^2)\circ T(p^{2l-2})$,
$T(p^{2l})$ and $T(p^{2l-4})$ and comment on it.
