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>