fixup! erp5_interface_post: add preview tab on Internet Message Post

bt5/erp5_interface_post/SkinTemplateItem/portal_skins/erp5_interface_post/ERP5Site_getFirstPartFromMultipartMessage.py

 """
 This script can be used to give a preview of an Internet Mail Message.
 Usually, the data is a multipart message (at least ERP5 only create
-multipart message). As for users don't want a fidel representation of
+multipart message). As for users don't want an accurate representation of
 the message, but a preview, we assume that rendering the first part
 of the multipart message is enough.
 """