[FIX] mass_mailing: include the correct file reference in the demo data

Currently in mass mailing, the logo is missing in template id "mass_mail_1" of
the demo data. This is because there is a wrong file reference in the template.

With a recent refactoring (see commit[1]), it renames the "s_default_image_logo"
with "s_default_image_header_logo" but not change the file reference in
the demo data.

This commit resolves the above issue by including the correct file reference
in the demo data template.

commit[1] -https://github.com/odoo/odoo/commit/a559fe4
task-3054248

closes odoo/odoo#105817

X-original-commit: 9711103632521d111d7ce4e179d298a95d32e759
Related: odoo/enterprise#33972
Signed-off-by: Stéphane Debauche (std) <std@odoo.com>

addons/mass_mailing/data/mass_mailing_demo.xml

@@ -96,7 +96,7 @@
                             <div class="row">
                                 <div valign="center" width="30%" class="col text-center o_mail_v_padding pb0" style="padding:20px 0 0px 0;vertical-align:middle;text-align:center;">
                                     <a href="http://www.example.com" style="text-decoration:none;font-weight:bold;background-color:transparent;color:rgb(100, 89, 116);">
-                                        <img border="0" src="/mass_mailing/static/src/img/theme_default/s_default_image_logo.png" style="border-style:none;height:auto;vertical-align:middle;max-width:400px;width:auto"/> ​
+                                        <img border="0" src="/mass_mailing/static/src/img/theme_default/s_default_image_header_logo.png" style="border-style:none;height:auto;vertical-align:middle;max-width:400px;width:auto"/> ​
                                     </a>
                                 </div>
                             </div>