Here we go.  There are 2 nodes circled above: the Diffuse node and a math node which actually controls gamma correction for the eyewhites.  Decreasing the Diffuse Node value will make the whites less brighter but we impart a greyish cast to them.  Maybe something you don't want.  My recommendation is to simply adjust the gamma node to 1 and re-render the image. Gamma correction is brightening the texture excessively.  One should restore the balance.