clean up a call to apr_psprintf() (trailing NULL not needed)

previously:
mod_auth_digest.c: In function `set_shmem_size':
mod_auth_digest.c:625: warning: too many arguments for format


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88275 13f79535-47bb-0310-9956-ffa450edef68