Fix undefined 'backend' in IBMQ getting-started example

[neonlight0123]

Fix undefined backend in the IBMQ getting-started example

In pytket/docs/getting_started.md, the IBMQ example declares a nairobi_device but then calls backend.run_circuit(...) — a variable that is not defined in that code block:

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)   # `backend` is undefined here

Running the guide top-to-bottom makes it NameError, or worse, silently runs the IBMQ-compiled circuit on the AerBackend that leaks from the earlier Aer example. The intended execution target is nairobi_device:

result = nairobi_device.run_circuit(compiled_circ, n_shots=100)

Verification:

• Confirmed on current main (2d03cce) that backend is undefined in this code block and nairobi_device is the only backend in scope.
• Confirmed the installed pytket.extensions.qiskit.IBMQBackend exposes both get_compiled_circuit and run_circuit, so the fix is valid API usage.
• No other backend misuse appears elsewhere in getting_started.md (the Aer example above legitimately defines backend = AerBackend()).

Doc-only change; no code, no version bump required.