What this says is that you find the minimal SoP expression for the F' expression and then negate it to find the NOR circuit.If you want a NOR circuit, you have to work with the F' (maxterm) representation:
F=(F')'=(BD+BC)'
=(BD)'(BC)'
=(B'+D')(B'+C')
Now you double negate this form and get what you wish:
F=(((B'+D')(B'+C'))')'
=((B'+D')'+(B'+C')')'
=(B' NOR D') NOR (B' NOR C')
which is a NOR circuit.