Fix a typo in pkinit.rst

(cherry picked from commit 231c4433f24470533a79cdd1743837b976970aee)

https://github.com/krb5/krb5/commit/aa5dac698c12e947fd7f53ed8daa49082bf8f966
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: aa5dac698c12e947fd7f53ed8daa49082bf8f966
Branch: krb5-1.11
 doc/admins/pkinit.rst |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)