changed target value to fix issue with 0.14.x typedoc

[mcherryleigh]

While using your plugin with typedoc 0.14.2 I was getting TypeError: Class constructor ConverterComponent cannot be invoked without 'new' which I resolved by cloning your repo and creating the dist locally with es6 as the target. This seems like it's probably related to TypeStrong/typedoc#943

[behaviary]

Thanks @mcherryleigh! I'm pointing my local at your fork till this PR gets merged :)

[behaviary]

For people who randomly find this in the cold, far-flung future. There is now an integration with docusaurus directly on the original typedoc-plugin-markdown: https://www.npmjs.com/package/typedoc-plugin-markdown#arguments