I have been trying to solve Rubik's cube using beginner's method provided here: https://ruwix.com/the-rubiks-cube/how-to-solve-the-rubiks-cube-beginners-method/
I ended up with just one last corner which is not oriented properly as shown in the picture. I cannot seem to solve it using R’ D’ R D algorithm, it messes up the whole square when the corner piece gets oriented. Every time I try to solve the cube, it again has just one last corner disoriented. None of the algorithms mention one last corner case. They always talk about 2 or more corners disoriented. I don't understand. Can someone explain what's happening and how to solve it?