Bind keys of lesson documentation for loop

aleksis/apps/alsijil/static/js/vue/components/alsijil/CourseBook.js

@@ -150,8 +150,8 @@ export default {
     <div>
       <v-subheader>{{ lesson.subject.name }}</v-subheader>
       <v-row>
-        <v-col v-for="item in lesson_documentations" cols="12"">
-          <lesson-documentation
+        <v-col v-for="item in lesson_documentations" cols="12" v-bind:key="item.id">
+          <lesson-documentation v-bind:key="item.id" :groups="lesson.groups"
              :period="item.period_formatted" :personal-notes="item.personal_notes"
             :date="item.date" :topic="item.topic" :homework="item.homework" :group-note="item.group_note"
           ></lesson-documentation>