2 in fact. Either bevel the edge with ctrl B, and flatten it out, or use the knife tool. k to begin, cut from vertex to vertex on either side, using enter to confirm. Then simply delete the top corner and fill in to create the flat plane.
I did the cut and deleted the things above, but somehow its not filled and when i was trying to place a plate there, it wouldnt fit.
EDIT: FILLED WITH F, WHAT AN OBVIOUS THING LOL
Certainly the best and fastest way. How did that slip my mind.
Select the two verts on the side and press J to create a diagonal cut between them. Then do the same on the other side. Delete the unwanted faces and select the edges that form the open end. Press F to fill the face in.