docs: The method optimize_for_inference doesn't return anything

[nok]

Update example code in the README.md, because the method optimize_for_inference doesn't return anything.

Current:

model = RFDETRBase()
model = model.optimize_for_inference()
# ...
detections = model.predict(image, threshold=0.5)

Fixed:

model = RFDETRBase()
model.optimize_for_inference()  # just call the method
# ...
detections = model.predict(image, threshold=0.5)

This fix is related to the issue #237.

[isaacrob-roboflow]

resolved with this merge #283