Grab the Red Eye Tool from the tool panel. You can also do it by pressing Shift+J again and again.
Now we need to define the pupil size and the darkness in the option panel.
By default, value of both is 50% but we can alter them.
Pupil size here can be defined by the percentage of the red color that is inside the pupil. If it 50%, it means that the size of the red color is 50% or half the size of the pupil. In my case, the red color and the pupil is of same size, so I am going to put the size of the pupil as 100%.
By darkness, what Photoshop means that the amount by which Photoshop should darken the pupil after removing the red color. If you want nice black pupil, you can set the darkness to 100% but I think it is too much. Usually I never go beyond 70%. For this image, I am going with 62%.