Update config_spec.md
Update config_spec.md

Update config documentation of `interp_order`

Please refer to https://github.com/NifTK/NiftyNet/blob/a383ba342e3e38a7ad7eed7538bfb34960f80c8d/niftynet/layer/rand_elastic_deform.py#L93 and issue https://github.com/NifTK/NiftyNet/issues/138  for details.
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