[FIX] snailmail: Fix pdf render function call

Fixes a small but massive typo that prevents snailmail from working on
version 14 to 15

closes odoo/odoo#105707

Signed-off-by: Florian Daloze (fda) <fda@odoo.com>

addons/snailmail/models/snailmail_letter.py

@@ -143,7 +143,7 @@ class SnailmailLetter(models.Model):
             paperformat = report.get_paperformat()
             if (paperformat.format == 'custom' and paperformat.page_width != 210 and paperformat.page_height != 297) or paperformat.format != 'A4':
                 raise UserError(_("Please use an A4 Paper format."))
-            pdf_bin, unused_filetype = report.with_context(snailmail_layout=not self.cover, lang='en_US').render_qweb_pdf(self.res_id)
+            pdf_bin, unused_filetype = report.with_context(snailmail_layout=not self.cover, lang='en_US')._render_qweb_pdf(self.res_id)
             if self.cover:
                 pdf_bin = self._append_cover_page(pdf_bin)
             attachment = self.env['ir.attachment'].create({