message: Change --output_png to --output_image
  The argument is incorrect.
  When running the given command, we get
  ```
  E tensorflow/examples/wav_to_spectrogram/main.cc:54] Unknown argument
  output_png=/tmp/spectrogram.png
  ```
  TESTED:Rerun the updated command and verify that the flag is correct.
  ```
  bazel run tensorflow/examples/wav_to_spectrogram:wav_to_spectrogram --
  input_wav=/tmp/speech_dataset/happy/ab00c4b2_nohash_0.wav
  output_image=/tmp/spectrogram.png
  ```
diff: @@ -280,7 +280,7 @@ tool:
  ```
  bazel run tensorflow/examples/wav_to_spectrogram:wav_to_spectrogram -- \
  --input_wav=/tmp/speech_dataset/happy/ab00c4b2_nohash_0.wav \
  ---output_png=/tmp/spectrogram.png
  +--output_image=/tmp/spectrogram.png
  ```
  If you open up `/tmp/spectrogram.png` you should see something like this:
  