fix: Mention GetBucketyPolicy in S3 gateway docs policy (#12642)

That action is needed for the gateway to obtain the policy
setting for public access.

Resolves: https://github.com/minio/minio/issues/12638

docs/gateway/s3.md

@@ -54,6 +54,7 @@ Minimum permissions required if you wish to provide restricted access with your
             "Sid": "readonly",
             "Effect": "Allow",
             "Action": [
+                "s3:GetBucketPolicy",
                 "s3:HeadBucket",
                 "s3:ListBucket"
             ],