Change example module name in host_config.rst

Don't use "otp" as the example clpreauth module name in
host_config.rst, since we now ship an effectively built-in otp
clpreauth module.  Instead use "mypreauth".

(cherry picked from commit 26a43afbc214e8d24cf261f245007192eccb7ed5)

https://github.com/krb5/krb5/commit/bcc00cc1a032cb952eb923d622b046bb949741fa
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: bcc00cc1a032cb952eb923d622b046bb949741fa
Branch: krb5-1.12
 doc/admin/host_config.rst |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)