There's so little code in that factory, I don't really understand the point of making it a Class.
app = angular.module("app", ["ngResource", "ngRoute"])
app.factory "Notes", ["$resource", ($resource) ->
$resource("/notes/:id", null, { update: { method: "PUT" } })
]
But having said that, if you want to use coffeescript classes for services in general (it doesn't work well with factories since they're singletons), you can do something like this:
class Foo
constructor: ($someDependency) ->
@myVar = "hello"
bar: ->
console.log @myVar
app.service("Foo", ['$someDependency', Foo])