hier(7): fix a couple mandoc warnings

These references are rendered correctly without the zero-width space.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

share/man/man7/hier.7

@@ -88,7 +88,7 @@ device special files managed by
 .It Pa fd/
 file descriptor files;
 see
-.Xr \&fd 4
+.Xr fd 4
 .El
 .It Pa /home/
 user HOME directories.
@@ -502,7 +502,7 @@ see
 .It Pa at/
 timed command scheduling files;
 see
-.Xr \&at 1
+.Xr at 1
 .Pp
 .Bl -tag -width Fl -compact
 .It Pa jobs/
@@ -580,7 +580,7 @@ see
 temporary home of files preserved after an accidental death
 of an editor;
 see
-.Xr \&ex 1
+.Xr ex 1
 .It Pa quotas/
 file system quota information files
 .It Pa run/