Update Custom Registries example code

[jeanbmar]

Task functions medata are lost when using fn.bind as in:

ConfigRegistry.prototype.set = function set(name, fn) {
  // The `DefaultRegistry` uses `this._tasks` for storage.
  var task = this._tasks[name] = fn.bind(this.config);
  return task;
};

Updating the example with the following would preserve metadata:

ConfigRegistry.prototype.set = function set(name, fn) {
  // The `DefaultRegistry` uses `this._tasks` for storage.
  var task = this._tasks[name] = Object.assign(fn.bind(this.config), fn);
  return task;
};

It's not a big deal but it would definitely save some times to devs wondering why metadata aren't displaying suddenly :).

[phated]

Good catch! Would you like to send a PR for this, I can get it merged and released on the website.

[jeanbmar]

PR #2565 sent!

[phated]

Thanks again!