Update detect.py (Fix save-csv: Ensure header is written to CSV) (#13472)

This commit resolves an issue where the save-csv command did not write the CSV header. The code now correctly saves the header in the CSV file.

Signed-off-by: Ali Ghanbari <alighanbari446@gmail.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>