fix event hook

[maxushuang]

Once an event, the event will not be removed. Because this event name become $$event in the off function.

[Akryum]

@maxushuang Why are you making this change?

[posva]

Hey sorry for the delay and thanks for the PR!
Could you explain a bit more what is not working as expected right now and is fixed by your PR?

[maxushuang]

Hey, sorry, reply delay.
The vue-devtools source in src/backend/hook.js is below.

const hook = {
    once (event, fn) {
      event = '$' + event
      function on () {
        this.off(event, on)
        fn.apply(this, arguments)
      }
      ;(listeners[event] || (listeners[event] = [])).push(on)
    },
   off (event, fn) {
      event = '$' + event
      if (!arguments.length) {
        listeners = {}
      } else {
     ......
};

hook.once('event1', _ => {});

hook.emit('event1');

When hook emits event1, hook will call this.off(event, on). But, now, the event in this.off has becomes $event1, hook will add a '$' symbol again when call off function.
So hook will not off event correctly.