Update wording of class order message

[adamhollett]

Description

This is a small nitpick — it changes the wording of the message when linting for class order.

• Changes from invalid to incorrect. The class order may be wrong but it's still valid CSS / Tailwind.
• Changes from classnames order to class order since the things being ordered are classes.

Type of change

(none of the listed options — just updates a single string)

How Has This Been Tested?

Nothing too extensive. I ran the test suite locally.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings