[FIX] l10n_be_intrastat: typo EXTRF now 29/19

Intrastat was modified on january 2019 to backport some features for new
regulation of belgian intrastat in c5a8e397.

There was a coding typo so EXTRF was <Report/> code attribute instead.

sources:
 https://www.nbb.be/doc/dq/e_pdf_ex/docxml-en.pdf
 https://www.nbb.be/doc/dq/f_pdf_ex/docxml-fr2019.pdf



opw-1948336
closes #31875

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>