Update typescript.md

[github0013]

webpacker 3.2.0 has drastic changes in the rules directory.

https://github.com/rails/webpacker/tree/v3.1.1/package/rules
https://github.com/rails/webpacker/tree/v3.2.0/package/rules

If you don't add ts-loader in the environment.js, you will end up with this error.

ERROR in ./app/javascript/packs/hello_react.tsx
Module parse failed: Unexpected token (10:11)
You may need an appropriate loader to handle this file type.
| class Hello extends React.Component {
|   render() {
|     return <div>hello world</div>;
|   }
| }
 @ multi (webpack)-dev-server/client?http://localhost:3035 ./app/javascript/packs/hello_react.tsx

[corymcdonald]

Hey there! I actually added an installer for typescript in this PR - #1145

I think this might fix this issue