Properly sort Xr to silence mandoc warnings.

Differential Revision:	https://reviews.freebsd.org/D1314
Reviewed by:	wblock
MFC after:	1 Month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275813 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f