Fix pnpm support in Sanity example

[stipsan]

As @sanity/preview-url-secret wasn't declared as a dependency when doing:

pnpm install && pnpm build

It would fail to compile:

Linting and checking validity of types  .Failed to compile.

./app/api/draft/route.tsx:6:36
Type error: Cannot find module '@sanity/preview-url-secret' or its corresponding type declarations.

  4 |  */
  5 |
> 6 | import { validatePreviewUrl } from "@sanity/preview-url-secret";
    |                                    ^
  7 | import { draftMode } from "next/headers";
  8 | import { redirect } from "next/navigation";
  9 |