Fix typescript error in sample app.

[rainshen49]

Not sure how this occurred given we locked down all dependencies

Type 'DocumentData' is missing the following properties from type 'Task': id, title, completed, owner, createdTime [plugin angular-compiler]

    src/app/services/task.service.ts:209:4:
      209 │     return collectionData(subtaskQuery, { idField: 'id' });