fix namespace and use spelling mistake

apps/backend/app/Mail/VolunteerUnassignToTaskMail.php

@@ -1,8 +1,8 @@
 <?php
 
-namespace app\Mail;
+namespace App\Mail;
 
-use app\Services\FormatDate;
+use App\Services\FormatDate;
 use Illuminate\Bus\Queueable;
 use Illuminate\Mail\Mailable;
 use Illuminate\Mail\Mailables\Content;