Fix a typo in Chunk#split jsdoc comment

Some history:
* `uglifyjs-webpack-plugin` < v1.0 used the minifier `uglify-js`
* however `uglify-js` does not support ES6, which resulted in a fork
  called `uglify-es` that was developed in the `uglify-js` repository,
  but under the `harmony` branch
* `uglifyjs-webpack-plugin` v1.x switched to `uglify-es` for ES6 support
* however `uglify-es` stopped being maintained:
  mishoo/UglifyJS#3156 (comment)
* which led to a fork called `terser` that has incorporated all of
  the unmerged PRs and will be where all new development occurs:
  https://github.com/fabiosantoscode/terser
* `terser-webpack-plugin` was created, which is the `terser` equivalent
  of `uglifyjs-webpack-plugin`:
  https://github.com/webpack-contrib/terser-webpack-plugin
* `uglifyjs-webpack-plugin` v2.x will be switching back to `uglify-js`,
  so any project that needs to support ES6 now needs to switch to
  `terser-webpack-plugin`.

Fixes #7923.

Cherry-pick terser to webpack 4