Fixing typo in README

[claylevering]

Just fixed the yml -> yaml typo in the example code 👍

[rtluckie]

@claylevering Welcome and thanks for the pull request! Gonna merge this PR, but I wanted to ask your opinion on yaml vs yml extension. YAML faqs ask that users "Please use '.yaml' when possible.", but I have come across several implementations that ignore this request (travis) and require the yml extension.
This is a completely immaterial question, but I just like hearing other's opinions on such matters. 😏

[claylevering]

I feel like this is a difference between 'file format' and 'name' and an inevitable consequence of old-school 8.3 Windows users. I think of 'mpeg' vs 'mpg'. It only becomes complicated, in my opinion, when the full-name comes dangerously close to three letters and could therefore also become an extension. You don't ever see an MKV file with a .Matroska extension.

Also, I figure if Travis ignores it, that's a fairly solid ground for dismissal of the 'rule'.