Bug 36140: Correct label for attribute invoice.tt

Bug 36140: Correct label for attribute invoice.tt

To test:
1. Go to /cgi-bin/koha/acqui/invoice.pl?invoiceid=X  where X is the invoice number.
2. Look at the Invoice number with your dev tools, the for attribute is wrong. ( shippingdate )
3. Apply patch and look again, the attribute should correctly be invoicenumber.

Alternatively, just look at the diff of this trivial change. :)

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>