Update Readme: terminal_state_cost() parameter includes action

[28shambhavi]

The Readme says terminal_state_cost is only a function of the full state: "terminal state cost- function(state (K x T x nx)) -> cost (K x 1)"

But in file src/mppi.py, line 276: "c = self.terminal_state_cost(states, actions)" gives two inputs to the function