Fix undefined 'backend' in IBMQ getting-started example

The IBMQ example declares 'nairobi_device' but then calls
'backend.run_circuit(...)', which is not defined in that code block (it
happens to resolve only by leaking the AerBackend from the earlier Aer
example). Run the compiled circuit on 'nairobi_device' instead.

Author: neonlight0123

pytket/docs/getting_started.md

@@ -117,7 +117,7 @@ nairobi_device = IBMQBackend('ibm_nairobi')
 
 # Compile Circuit to use supported gates of IBMQ Nairobi
 compiled_circ = nairobi_device.get_compiled_circuit(circ)
-result = backend.run_circuit(compiled_circ, n_shots=100)
+result = nairobi_device.run_circuit(compiled_circ, n_shots=100)
 ```
 
 Here the default compilation pass is applied by `IBMQBackend.get_compiled_circuit`. See [this page](https://docs.quantinuum.com/tket/extensions/pytket-qiskit/#default-compilation) for more details.