Property Units documentation citing deprecated naming rule

[adrifmonte]

I was looking into every rule documentation, and I found an incorrect naming usage:

https://github.com/sasstools/sass-lint/blob/master/docs/rules/property-units.md

While the options are just global and per-property, the second example mentions about globally-allowed-units and units-allowed-for-properties.

I believe those are their former names, which after a rename refactoring, was missed to be updated. Updating them to global and per-property should fix the issue.

[DanPurdy]

Thanks @adrifmonte, do you want to submit a PR? We can update it if not. 👍