Second order QCD corrections to the forward-backward asymmetry in e^+e^- - collisions

The sentence, 11th line below Eq. (30), starting with "Further in [14] one has included all interference terms ...." is wrong and has been corrected. Second line below Eq. (40) m_bar(m)=m is replaced by m_bar(\mu_0)=\mu_0 with \mu_0=4.10 GeV, \mu_0=1.30 GeV and \mu_0=166.1 GeV for bottom, charm and top respectively. The numbers in the second and third column of table 3 are a little bit changed. In the acknowledgement the funding agency is mentioned and one reference i.e. [20] is added.