Documentation fix for cirq.Circuit.prev_moment_operating_on (#4895)

See #4894 concerning a single-word documentation error for `cirq.Circuit.prev_moment_operating_on`

cirq-core/cirq/circuits/circuit.py

@@ -349,7 +349,7 @@ def prev_moment_operating_on(
         end_moment_index: Optional[int] = None,
         max_distance: Optional[int] = None,
     ) -> Optional[int]:
-        """Finds the index of the next moment that touches the given qubits.
+        """Finds the index of the previous moment that touches the given qubits.
 
         Args:
             qubits: We're looking for operations affecting any of these qubits.