Fix the yaml language ID in readme

[swift502]

"yml" appears to be an invalid ID since e169d64

Hope it's alright 👌

[Jason3S]

@swift502,

Thank you for the PR.

I do not think yml is a file type in VS Code, only yaml. Are you seeing a problem?

[Jason3S]

@swift502,

Thank you. I misunderstood your PR. I hadn't realized that you are changing the text in the README.md from yml to yaml.