pathchk(1): Fix the example so it allows arbitrary pathnames.

Spaces and various other characters in pathnames are not passed through
literally by xargs in its default mode. Instead, use find . -exec ... {} +

Although the -- argument is not strictly required here, add it anyway to
avoid surprises when modifying the code to find -f -somedir ...

MFC after:	1 week

usr.bin/pathchk/pathchk.1

@@ -114,7 +114,7 @@ other
 .Tn POSIX
 systems:
 .Pp
-.Dl "find . -print | xargs pathchk -p"
+.Dl "find . -exec pathchk -p -- {} +"
 .Sh SEE ALSO
 .Xr getconf 1 ,
 .Xr pathconf 2 ,