Hotfix for missing application/json header in Prepr example (#55517)

I added the application/json header to the API lib of the Prepr example.
Because without it when statically generating an error is produced by
GraphQL.

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

examples/cms-prepr/lib/api.js

@@ -2,6 +2,7 @@ async function fetchAPI(query, { variables, preview } = {}) {
   const response = await fetch(process.env.PREPRIO_API, {
     method: 'POST',
     headers: {
+      'Content-Type': 'application/json',
       Authorization:
         'Bearer ' +
         (preview