updated readme

Author: chiamp

flax/experimental/nnx/README.md

@@ -142,7 +142,7 @@ NNX Modules are normal python classes, they obey regular python semantics such a
 
 ```python
 class Foo(nnx.Module):
-  def __init__(self, rngs: nnx.Rngs):
+  def __init__(self, *, rngs: nnx.Rngs):
     # node attributes
     self.param = nnx.Param(jnp.array(1))
     self.submodule = nnx.Linear(12, 3, rngs=rngs)