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

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>