Harmonize Quickstart example (#717)

README.md

@@ -75,9 +75,10 @@ state = nuts.init(initial_position)
 
 # Iterate
 rng_key = jax.random.key(0)
+step = jax.jit(nuts.step)
 for step in range(100):
     nuts_key = jax.random.fold_in(rng_key, step)
-    state, _ = nuts.step(nuts_key, state)
+    state, _ = step(nuts_key, state)
 ```
 
 See [the documentation](https://blackjax-devs.github.io/blackjax/index.html) for more examples of how to use the library: how to write inference loops for one or several chains, how to use the Stan warmup, etc.