[fix] incorrect path to generated tsconfig.json (#4179)

When attempting to update my existing sveltekit project I  added `"extends": ".svelte-kit/tsconfig.json"` like the docs say and was met with the error `'.svelte-kit/tsconfig.json' not found.` I checked the template and the correct path was `./.svelte-kit/tsconfig.json`. Without the `./`, the project fails to build and every svelte file has an error.

documentation/docs/14-types.md

@@ -73,4 +73,4 @@ export async function get({ params }) {
 
 > For this to work, your own `tsconfig.json` or `jsconfig.json` should extend from the generated `.svelte-kit/tsconfig.json`:
 >
->     { "extends": ".svelte-kit/tsconfig.json" }
+>     { "extends": "./.svelte-kit/tsconfig.json" }