message: Update preview instructions
diff: >
  > To use preview APIs:
  >
  -> - You must use the preview version of the Office JavaScript API library from the [Office.js content delivery network (CDN)](https://appsforoffice.microsoft.com/lib/beta/hosted/office.js). The [type definition file](https://appsforoffice.microsoft.com/lib/beta/hosted/office.d.ts) for TypeScript compilation and IntelliSense is found at the CDN and [DefinitelyTyped](https://raw.githubusercontent.com/DefinitelyTyped/DefinitelyTyped/master/types/office-js-preview/index.d.ts). You can install these types with `npm install --save-dev @types/office-js-preview`.
  +> - You must use the preview version of the Office JavaScript API library from the [Office.js content delivery network (CDN)](https://appsforoffice.microsoft.com/lib/beta/hosted/office.js). The [type definition file](https://appsforoffice.microsoft.com/lib/beta/hosted/office.d.ts) for TypeScript compilation and IntelliSense is found at the CDN and [DefinitelyTyped](https://raw.githubusercontent.com/DefinitelyTyped/DefinitelyTyped/master/types/office-js-preview/index.d.ts). You can install these types with `npm install --save-dev @types/office-js-preview` (be sure to remove the types for `@types/office-js` if you've previously installed them).
  > - You may need to join the [Office Insider program](https://insider.office.com) for access to more recent Office builds.
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