Ideas on how to fix a void at the bottom edge of my concrete steps?
That's not a hole. That's just the bottom. Steps like this are poured on the dirt, and while the dirt level probably should have been flatter and lower (for aesthetic reasons), this isn't a structural flaw. 

Attempting to patch it will make things worse, visually speaking. Put some mulch over the bottom edge or raise the soil level a bit and be happy.