fix hamilton graph network example

[cpehle]

Without this fix I got the following error trace:

  File "jraph/examples/hamiltonian_graph_network.py", line 239, in hamiltonian_from_state_fn
    graph = set_system_state(static_graph, position, momentum)
  File "jraph/examples/hamiltonian_graph_network.py", line 202, in set_system_state
    nodes = static_graph.nodes.copy(position=position, momentum=momentum)
jax._src.traceback_util.UnfilteredStackTrace: TypeError: frozendict.copy() got an unexpected keyword argument 'position'

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