mtree: Update mtree flags in README file

- Add -b (suppress blank lines before directories).
- The equivalent of `-i` in fmtree is `-j` in mtree (nmtree) (indent the
  output 4 spaces).
- Add `-F freebsd9` compatibility flavor (print the closing `..` at the
  end).

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/912

etc/mtree/README

@@ -23,8 +23,8 @@ and keep them in good working order.
 
             mkdir /tmp/MTREE
             mtree -deU -f BSD.X.dist -p /tmp/MTREE
-            mtree -cdin -k uname,gname,mode -p /tmp/MTREE | \
-		sed -e 's/ *$//' >BSD.X.new
+            mtree -bcdjn -F freebsd9 -k uname,gname,mode -p /tmp/MTREE | \
+                sed -e 's/ *$//' >BSD.X.new
             diff -u BSD.X.dist BSD.X.new
             rm -r /tmp/MTREE