message: Fix plaidml setup instruction for build.md
  It seems the path where the .whl files are saved changed.
  This mini PR fixes the path mentioned in the `build.md`.
  In case this is just a quirk of my setup (not sure tbh), feel free to ignore
diff: @@ -87,7 +87,7 @@ bazelisk build //plaidml:wheel
  ## Install the PlaidML Python wheel
  ```
  -pip install -U bazel-bin/plaidml/*.whl
  +pip install -U bazel-bin/plaidml/wheel.pkg/dist/*.whl
  plaidml-setup
  ```
  @@ -182,7 +182,7 @@ bazelisk build //plaidml/keras:wheel
  ## Install the PlaidML-Keras Python wheel
  ```
  - pip install -U bazel-bin/plaidml/keras/*.whl
  + pip install -U bazel-bin/plaidml/keras/wheel.pkg/dist/*.whl
  ```
  # Testing PlaidML
  