Fix errors in Quantum algorithm

DIRECTORY.md

@@ -684,6 +684,7 @@
   * [Validate Solutions](https://github.com/TheAlgorithms/Python/blob/master/project_euler/validate_solutions.py)
 
 ## Quantum
+  * [Not Gate](https://github.com/TheAlgorithms/Python/blob/master/quantum/not_gate.py)
   * [Single Qubit Measure](https://github.com/TheAlgorithms/Python/blob/master/quantum/single_qubit_measure.py)
 
 ## Scheduling

quantum/not_gate.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python3
 """
-Build a simple bare-minimum quantum circuit that starts with a single 
-qubit (by default, in state 0) and inverts it. Run the experiment 1000 
+Build a simple bare-minimum quantum circuit that starts with a single
+qubit (by default, in state 0) and inverts it. Run the experiment 1000
 times and print the total count of the states finally observed.
 Qiskit Docs: https://qiskit.org/documentation/getting_started.html
 """
@@ -11,11 +11,13 @@
 
 def single_qubit_measure(qubits: int, classical_bits: int) -> q.result.counts.Counts:
     """
-    >>> single_qubit_measure(1, 1)
+    >>> single_qubit_measure(2, 2)
     {'11': 1000}
+    >>> single_qubit_measure(4, 4)
+    {'0011': 1000}
     """
     # Use Aer's qasm_simulator
-    simulator = q.Aer.get_backend('qasm_simulator')
+    simulator = q.Aer.get_backend("qasm_simulator")
 
     # Create a Quantum Circuit acting on the q register
     circuit = q.QuantumCircuit(qubits, classical_bits)
@@ -34,6 +36,6 @@ def single_qubit_measure(qubits: int, classical_bits: int) -> q.result.counts.Co
     return job.result().get_counts(circuit)
 
 
-if __name__ == '__main__':
+if __name__ == "__main__":
     counts = single_qubit_measure(2, 2)
-    print(f'Total count for various states are: {counts}')
+    print(f"Total count for various states are: {counts}")