validator: T2417: bugfix on Python3 f'ormat strings
validator: T2417: bugfix on Python3 f'ormat strings

Commit 3639a5610b590a ("validator: T2417: try to make the code clearer")
introduced Python3 f'ormatted strings but missed the "f" keyword.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x