fix the core dump bug of DS2's training in docker

Author: Yibing Liu

deep_speech_2/data_utils/featurizer/audio_featurizer.py

@@ -141,7 +141,8 @@ def _specgram_real(self, samples, window_size, stride_size, sample_rate):
  # window weighting, squared Fast Fourier Transform (fft), scaling
  weighting = np.hanning(window_size)[:, None]
  fft = np.fft.rfft(windows * weighting, axis=0)
- fft = np.absolute(fft)**2
+ fft = np.absolute(fft)
+ fft = fft ** 2
  scale = np.sum(weighting**2) * sample_rate
  fft[1:-1, :] *= (2.0 / scale)
  fft[(0, -1), :] /= scale