hier(7): fix inline markup for a variable

I mistakenly preceded macro Va with macro It in
https://github.com/freebsd/freebsd-src/pull/763/commits/8ff26f2cb5c0d69cce319e1ac1f792ea4245445a

https://bugs.freebsd.org/261349

PR: 261349
Approved by: emaste, imp, mhorne
Fixes: 6469f9c hier(7): improvement, modernisation