man filesystems: fix xrefs after move to section 4

Reviewed by: des, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1077

lib/libc/posix1e/posix1e.3

@@ -46,7 +46,7 @@ its interfaces are widely used.
 implements POSIX.1e interface for access control lists, described in
 .Xr acl 3 ,
 and supports ACLs on the
-.Xr ffs 7
+.Xr ffs 4
 file system; ACLs must be administratively enabled using
 .Xr tunefs 8 .
 .Pp
@@ -95,7 +95,7 @@ for mandatory access control labels.
 .Xr libcasper 3 ,
 .Xr mac 3 ,
 .Xr capsicum 4 ,
-.Xr ffs 7 ,
+.Xr ffs 4 ,
 .Xr getfmac 8 ,
 .Xr setfmac 8 ,
 .Xr tunefs 8 ,