Documentation/git-archive: spell --worktree-attributes correctly

The --worktree-attributes option was correctly documented in ba053ea
(archive: do not read .gitattributes in working directory,
2009-04-18).  However, later in 9b4c8b0 (archive documentation:
attributes are taken from the tree by default, 2010-02-10) the
misspelling "--work-tree-attributes" was used to refer to it.  Fix
this.

Noticed-by: Jeffrey Phillips Freeman <jeffrey.freeman@syncleus.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-archive.txt

@@ -116,7 +116,7 @@ Note that attributes are by default taken from the `.gitattributes` files
 in the tree that is being archived.  If you want to tweak the way the
 output is generated after the fact (e.g. you committed without adding an
 appropriate export-ignore in its `.gitattributes`), adjust the checked out
-`.gitattributes` file as necessary and use `--work-tree-attributes`
+`.gitattributes` file as necessary and use `--worktree-attributes`
 option.  Alternatively you can keep necessary attributes that should apply
 while archiving any tree in your `$GIT_DIR/info/attributes` file.