[FIX] ir.mail_server: fix bug introduced in 200612ca1e

The variable `email_body_utf8` has been renamed to `email_body` during
PythonThreeTization but there is a leftover:

https://github.com/odoo/odoo/commit/200612ca1ef1#diff-85eeeed32c7f2fe285884f3661b49881R326

odoo/addons/base/ir/ir_mail_server.py

@@ -323,7 +323,7 @@ class IrMailServer(models.Model):
 
         if subtype == 'html' and not body_alternative:
             # Always provide alternative text body ourselves if possible.
-            text = html2text.html2text(email_body_utf8)
+            text = html2text.html2text(email_body)
             alternative_part = MIMEMultipart(_subtype="alternative")
             alternative_part.attach(MIMEText(text, _charset='utf-8', _subtype='plain'))
             alternative_part.attach(email_text_part)