Please note the negative sign. This means that the system is bounded, i.e, the nucleus attracts the electrons and hence the energy of the system is negative. But as we move further away from the nucleus, this energy becomes less negative. That is to say, the nucleus' force of attraction on the electron decreases. When n approaches infinity, the energy of the system = 0. The electron is now free. So, technically speaking, the energy is in fact, increasing.