Add missing translatable string

aleksis/apps/alsijil/templates/alsijil/class_register/person.html

@@ -88,7 +88,7 @@
               {% if can_mark_all_as_excused %} medium-high-right {% endif %}"
               data-target="filter-modal"
               type="button">
-            Filter results ({{ num_filters }})<i class="material-icons right">filter_alt</i>
+            {% trans "Filter results" %} ({{ num_filters }})<i class="material-icons right">filter_alt</i>
           </button>
         </div>
         <form action="" method="post" class="">