add item updates to the todo-app example

[chrisvfritz]

Fixes #36.

[posva]

Shouldn't we do ref.child().set() like this: https://jsfiddle.net/posva/wtyop5jy/ ?
update looks a bit dangerous for one field only, doesn't it?
I think you got it from the Samples in the docs (https://github.com/firebase/quickstart-js/blob/master/database/scripts/main.js)
@jwngr What are your thoughts on this? Should we show the set version on the example and add a link to the Reference doc (https://firebase.google.com/docs/reference/js/firebase.database.Reference)

[jwngr]

Yeah I would use set() on the deep child path instead of update() on the parent node.

[chrisvfritz]

Thanks for the feedback! Updated.