Fix tf-keras installation error

[ceme393]

Description

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  using the provided document lead to error of tf_keras module not being found. I added installation of tf-keras package in the instructions which fixes it.

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• [ .] Documentation update
• TensorFlow 2 migration
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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Tests

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  try running python yamnet_test.py without first installing tf-keras package and then after installing it.
  Test Configuration:

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[plakal]

Thanks for the fix! I could swear these installation instructions worked without having to install tf-keras when I last updated the README, because keras is included in tensorflow. It's possible they have been separated since then.