btrfs-progs-unstable: replace debug-tree to btrfs-debug-tree in INSTALL

From c04da1655df6d75db834ddbd3a3b4a58a0d9a0c9 Mon Sep 17 00:00:00 2001
From: Wang Sheng-Hui <shhuiw@gmail.com>
Date: Mon, 18 Jul 2011 02:17:31 -0500
Subject: [PATCH] btrfs-progs-unstable: replace debug-tree to btrfs-debug-tree in INSTALL

debug-tree doesn't exist after btrfs-progs installed.
Use btrfs-debug-tree to print FS metadata in text form,
not debug-tree.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>