fix(projectHistoryLogs): incorrect message on ph log export email TAS…

…K-1486 (#5450)

### 📣 Summary
Update subject line for project history log export email

### 👀 Preview steps
Easiest to test this within kobo-compose so you can see emails in the
kpi logs.

Bug template:
1. ℹ️ have an account and a project
2. Go to `api/v2/asset/<uid>/history/export`
3. 🔴 [on main] The email subject is in title case
4. 🟢 [on PR] The email subject is in sentence case

kpi/models/import_export_task.py

@@ -605,7 +605,7 @@ class ProjectHistoryLogExportTask(
 
     @property
     def default_email_subject(self) -> str:
-        return 'Project History Log Report Complete'
+        return 'Project activity log export complete'
 
     def get_data(self, filtered_queryset: QuerySet) -> QuerySet:
         return filtered_queryset.annotate(