Fix typo on working directory (tree)

As per #1559:
nothing to commit, working directory -> tree

Signed-off-by: Bagas Sanjaya <[email protected]>

Author: bagasme

book/02-git-basics/sections/recording-changes.asc

@@ -27,7 +27,7 @@ If you run this command directly after a clone, you should see something like th
 $ git status
 On branch master
 Your branch is up-to-date with 'origin/master'.
-nothing to commit, working directory clean
+nothing to commit, working tree clean
 ----
 
 This means you have a clean working directory; in other words, none of your tracked files are modified.