It might just be a typo. From definition 3, [itex]\left( \left|0++\right>+i\left|1--\right> \right)/\sqrt{2} [\itex] is supposed to be the graph state [itex]\left| G \right>[\itex]. If I am correct in that assumption then [itex] \left|G\right> [\itex] cannot have any imaginary amplitudes. This is because you can create the graph by doing a series of controlled phases, one for each edge of the graph, to the state [itex]\left|+\right>^{\otimes 3}[\itex]. Controlled phases can only change the phase of a computational basis state by [itex]-1[\itex].