Update q_fourier_transform.py

[ridds-io]

Corrections Summary:

• Docstring cleaned up: More accurate explanation of input/output.
• Proper indexing in phase gate: The control and target qubits were set correctly (j, i).
• Removed unnecessary counter decrement.
• Aer backend fetching: Cleaned up.
• Error handling improvements: Slight refinement of error messages.
• Swaps and measurements: Logic is clearer and follows proper QFT construction.