Fix todomvc example: prefix todomvc mounted hook with "vue:"

[ethmcc]

The todomvc example has a use of the @mounted hook, triggering an error:

mounted and unmounted hooks now need to be prefixed with vue: - use @vue:mounted="handler" instead.

I checked for any other stray uses of @mounted or @unmounted in the repo to clean up; there were no other instances.

[ethmcc]

Closing because I looked through the open PRs and noticed that fixes for this and other examples were already proposed in: #224.

Looks like this bug first noted in #170, over two years ago, but nothing has been merged since months before that.

[jikkuatwork]

Thats sad! Wish the relevant fixes were addressed.