Update README.md fixed directml command "directml" to "dml"

[JcobCN]

In the latest version, it should be "dml".

Summary by Sourcery

Update the README.md to reflect the correct command for the execution provider, changing 'directml' to 'dml'.

Documentation:

• Correct the command in the README.md to use 'dml' instead of 'directml' for the execution provider.

[sourcery-ai[bot]]

Reviewer's Guide by Sourcery

This pull request updates the README.md file to correct the command for the execution provider from 'directml' to 'dml' in accordance with the latest version.

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