comparators.xml: fixes test_xml_expected_diff output encoding to UTF-8

The enconding update was made on a previous patch and reflects
the result in this updated diff.

Signed-off-by: Juliana Oliveira Rodrigues <juliana.orod@gmail.com>

tests/data/test_xml_expected_diff

 @@ -1,9 +1,9 @@
- <?xml version="1.0" ?>
+ <?xml version="1.0" encoding="utf-8"?>
  <note>
    <style id="night-mode-pro-style" type="text/css"/>
    <link id="night-mode-pro-link" rel="stylesheet" type="text/css"/>