chore(examples): require WORDPRESS_API_URL to include /graphql

[balazsorban44]

Fixes #38749

Since we are calling it API_URL, it sounds more correct to require including the /graphql rather than making WORDPRESS_API_URL be just the root domain in

next.js/examples/cms-wordpress/README.md

Line 96 in 54f8724

Then open `.env.local` and set `WORDPRESS_API_URL` to be the URL to your GraphQL endpoint in WordPress. For example: `https://myapp.wpengine.com/graphql`.

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