State diagram for JK-flip-flop
The state diagram is correct, but, for completeness, I would put (in the upper circle) Q = 0 and /Q = 1, and in the lower circle, Q = 1 and /Q = 0.

Why? Because if you want to add the effect of the reset and set entries to the JK FF (which most circuits have), then the extra states (Q = 0 and /Q = 0, and both at 1) are possible. 

But, if you simple consider the basic JK, then your diagram is correct.