fix: check the qubit set length against observables (#970)

src/braket/circuits/result_type.py

@@ -221,7 +221,7 @@ def __init__(
                     "target length is equal to the observable term's qubits count."
                 )
             self._target = [QubitSet(term_target) for term_target in target]
-            for term_target, obs in zip(target, observable.summands):
+            for term_target, obs in zip(self._target, observable.summands):
                 if obs.qubit_count != len(term_target):
                     raise ValueError(
                         "Sum observable's target shape must be a nested list where each term's "