documentation update - use http://minio-eu:9000 for consistency

Hi, Team !

Could you pls. update the [page](https://cloudnative-pg.io/plugin-barman-cloud/docs/usage/#defining-the-objectstore) of the documentation the following way:

- old value
endpointURL: http://minio:9000

- proposed value
endpointURL: http://minio-eu:9000

This is for the consistency of the documentation.

Open the linked documentation page at cloudnative-pg.io/plugin-barman-cloud/docs/usage/#defining-the-objectstore and find the endpointURL example. Change the example from http://minio:9000 to http://minio-eu:9000, then verify the rendered page shows the proposed value consistently.

Written by the indexing model from the issue text.