docs: update the s3 client example for accesing RGW
docs: update the s3 client example for accesing RGW

Couple of modification to existing example for s3 client regarding how
to access the ceph object store. Added details to fetch the
`BUCKET_NAME`, updated the `AWS_ENDPOINT` and `AWS_HOST` details.

Signed-off-by: Jiffin Tony Thottan <ac134a614b50f2e879db85acb9f4b45b24a210dc@gmail.com>

rook/rook,rook/rook,rook/rook,rook/rook,rook/rook