message: Correct paths in file_mac README
diff: @@ -27,8 +27,8 @@ bazel build ...
  echo "some data" > data.txt
  echo "01293ce6590fb883aa111a53e56d537dea5641b664901e35ce10e0c31df8398218fa9b030d" > expected.txt
  -./bazel-bin/examples/file_mac/python/file_mac ./examples/file_mac/python/hmac_sha256_256bittag_test_keyset.json \
  +./bazel-bin/examples/python/file_mac/file_mac_cleartext ./examples/python/file_mac/hmac_sha256_256bittag_test_keyset.json \
  data.txt
  -./bazel-bin/examples/file_mac/python/file_mac ./examples/file_mac/python/hmac_sha256_256bittag_test_keyset.json \
  +./bazel-bin/examples/python/file_mac/file_mac_cleartext ./examples/python/file_mac/hmac_sha256_256bittag_test_keyset.json \
  data.txt expected.txt
  ```
  