refactor: edit heading in add.html in assignments app

Change from "add assignment" to "new assignment".

widget_group_6/assignments/templates/assignments/add.html

 {% extends "assignments/base.html" %}
 
 {% block content %}
-    <h1>Add Assignment</h1>
+    <h1>New Assignment</h1>
     <form method="POST" enctype="multipart/form-data" action="{% url 'assignments:add' %}">
         {% csrf_token %}
         {{assignment_form.media}}