fix hardcoded fieldname in taskDAO

app/src/main/java/be/ugent/sel/studeez/data/local/models/task/Task.kt

@@ -9,3 +9,11 @@ data class Task(
     val time: Int = 0,
     val subjectId: String = "",
 )
+
+object TaskDocument {
+    const val id = "id"
+    const val name = "name"
+    const val completed = "completed"
+    const val time = "time"
+    const val subjectId = "subjectId"
+}