I would be easier to use lasso (L) to select darker area. Be sure you select a little bit larger then shadow is
finally you can use Edit > Fill (Shift+F5) with Content Aware command
You can also use Spot Healing Brush with content aware (defoult) selected
Thanks a lot! Saved me lots of time. Although not a flawless solution, since it does change the content a bit, in my case it worked like a charm.