Fix tket error in document generation (#464)

- pytket recently restructured its extensions to make each submodules
so the cirq_to_tket function needs to be imported specifically from the
submodule.

Author: dstrain115

docs/qaoa/routing_with_tket.ipynb

@@ -241,9 +241,10 @@
    "outputs": [],
    "source": [
     "import pytket\n",
+    "from pytket.extensions.cirq import cirq_to_tk\n",
     "from recirq.qaoa.placement import _device_to_tket_device\n",
     "\n",
-    "tk_circuit = pytket.extensions.cirq.cirq_to_tk(circuit)\n",
+    "tk_circuit = cirq_to_tk(circuit)\n",
     "tk_device = _device_to_tket_device(device)"
    ]
   },