Use supported icons.

biscuit/apps/chronos/templates/chronos/lessons_day.html

@@ -11,10 +11,10 @@
  </div>
  <div class="btn-group" role="group" aria-label="Day actions">
   <a href="{% url 'lessons_day_by_date' day_prev %}" class="btn btn-secondary">
-   {% fa 'hand-point-right' %}
+   {% fa 'arrow-left' %}
   </a>
   <a href="{% url 'lessons_day_by_date' day_next %}" class="btn btn-secondary">
-   {% fa 'hand-point-left' %}
+   {% fa 'arrow-right' %}
   </a>
  </div>
 </div>