[FIX] project: fix line break issue in user assigneed mail template

In your inbox, when you are assigned to a task then you receive mail from
the project. In that mail, the mail subject and body are shown in one line.

In this commit, The mail template's "assignee_name" field currently has a 'span'
tag, which may cause inline subject and body issues. To prevent this, it is
recommended to use a 'div' tag instead.

task-3274690

closes odoo/odoo#118576

Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>

addons/project/data/mail_template_data.xml

@@ -103,10 +103,11 @@
 
         <!-- You have been assigned email -->
         <template id="project_message_user_assigned">
-    <span>Dear <t t-esc="assignee_name"/>,</span>
+<div>
+    Dear <t t-esc="assignee_name"/>,
     <br/><br/>
     <span style="margin-top: 8px;">You have been assigned to the <t t-esc="model_description or 'document'"/> <t t-esc="object.display_name"/>.</span>
-    <br/>
+</div>
         </template>
     </data>
 </odoo>