Change variable names (#654)

blackjax/mcmc/mala.py

@@ -79,7 +79,7 @@ def build_kernel():
     def transition_energy(state, new_state, step_size):
         """Transition energy to go from `state` to `new_state`"""
         theta = jax.tree_util.tree_map(
-            lambda new_x, x, g: new_x - x - step_size * g,
+            lambda x, new_x, g: x - new_x - step_size * g,
             state.position,
             new_state.position,
             new_state.logdensity_grad,