💚 Fix broken assertion in test

Jan 1st + 1 month equals Feb. 1st, not Jan 31st. Why was this test passing then, you ask?
Well, the PDF also includes the timestamp when it was generated, based on 'now'. The
PR with the fix (and test) was created on Jan 31st, effectively causing the expected
string to be present in the PDF. Until, of course, time passed and it became
February.

src/openforms/submissions/tests/test_tasks_pdf.py

@@ -147,7 +147,7 @@ def test_date_object_is_converted_to_str_when_it_comes_from_logic_rule(self):
 
         html = submission.report.generate_submission_report_pdf()
 
-        self.assertIn("31 januari 2025", html)
+        self.assertIn("1 februari 2025", html)
 
     def test_visible_output_included(self):
         """