Update TypeScript document to point to jsx runtime mode

[marvinhagemeister]

TypeScript supports the new runtime jsx transpilation now, which automatically adds imports like the babel plugin. It can be enabled by either setting jsx to react-jsx or react-jsx-dev.

See: https://www.typescriptlang.org/docs/handbook/jsx.html

The page for TypeScript can be found here: https://github.com/preactjs/preact-www/blob/master/content/en/guide/v10/typescript.md

Related issue: preactjs/preact#3180

[marvinhagemeister]

Err nevermind we have that already 🙈