git-add doc: do not say working directory when you mean working tree

The usage of working directory is inconsistent in the git add help.
Also http://git-scm.com/docs/git-clone

 speaks only about working tree.
Remaining entry found by "git grep -B1 '^directory' git-add.txt" really
relates to a directory.

Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -24,7 +24,7 @@ remove paths that do not exist in the working tree anymore.
 
 The "index" holds a snapshot of the content of the working tree, and it
 is this snapshot that is taken as the contents of the next commit.  Thus
-after making any changes to the working directory, and before running
+after making any changes to the working tree, and before running
 the commit command, you must use the `add` command to add any new or
 modified files to the index.