fix darwin platform string (WongKinYiu#454)

export.py

@@ -89,7 +89,7 @@
  ct_model = ct.convert(ts, inputs=[ct.ImageType('image', shape=img.shape, scale=1 / 255.0, bias=[0, 0, 0])])
  bits, mode = (8, 'kmeans_lut') if opt.int8 else (16, 'linear') if opt.fp16 else (32, None)
  if bits < 32:
- if sys.platform == 'Darwin': # quantization only supported on macOS
+ if sys.platform.lower() == 'darwin': # quantization only supported on macOS
  with warnings.catch_warnings():
  warnings.filterwarnings("ignore", category=DeprecationWarning) # suppress numpy==1.20 float warning
  ct_model = ct.models.neural_network.quantization_utils.quantize_weights(ct_model, bits, mode)