examples (todomvc): change @vnode-mounted to @vue:mounted (#2418)

This example was using a deprecated API (`@vnode-mounted`), so I updated it.

Author: jynxio

src/examples/src/todomvc/App/template.html

@@ -34,7 +34,7 @@ <h1>todos</h1>
           class="edit"
           type="text"
           v-model="todo.title"
-          @vnode-mounted="({ el }) => el.focus()"
+          @vue:mounted="({ el }) => el.focus()"
           @blur="doneEdit(todo)"
           @keyup.enter="doneEdit(todo)"
           @keyup.escape="cancelEdit(todo)"