They should have sand or round, non-sharp gravel. you could try adding some round pebble gravel on top of what you have or just replace the gravel completely. If you completely replace the gravel only do like half a week cuz the beneficial bacteria in the gravel.
Also the corys prefer to be in three+ schools and should really be in a bigger tank.
Good info above from NMfishman. You could also leave the gravel you have in there now & just add pool filter sand. The sand will settle in the gaps between the gravel & if you add enough sand to put a 1/4"-1/2" layer of the sand over the top of the gravel after the sand has settled, the corys won't be able to hurt thier barbels on the sharp gravel.
I have been very pleased with pool filter sand so far. The corys like it too. In fact they scoot about the surface far differently then they do with gravel. Pool Filter Sand has the added benefit of being cheap and relatively clean.
From what I've read it is bacteria in the substrate that causes barbel erosion and not sharp particles in the substrate. In any event, I (and the corys) are digging the sand.