Mention the chpass command as a way to change the language.

Add an example with a short description on how to use chpass to change the
language for an existing user.

The patch was provided by arrowd@, I merely did some grammar and indentation
cleanups.

PR: 238890
Submitted by: omen-54@ya.ru
Patch by: arrowd
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D20825