[IMP] account: more explicit email (with the design, it's not obvious that it's in attachment)

addons/account/data/email_template_data_invoice.xml

@@ -25,7 +25,7 @@
     (<i>${object.partner_id.parent_id.name}</i>)
 % endif
 ,</p>
-<p>Here is your 
+<p>Here is, in attachment, your 
 % if object.number:
 invoice <strong>${object.number}</strong>
 % else: