Update backup_restore.md documentation regarding S3 and IAM profiles.

https://gitlab.com/gitlab-org/gitlab-ce/commit/7f8ef19c411eceec207fef5d8459fbeaa40bf464 most likely broke the old configuration format where empty string aws_access_key_id & aws_secret_access_key were still OK. From 9.5 onwards moving backups to S3 using IAM profiles will fail if they are configured.

doc/raketasks/backup_restore.md

@@ -144,9 +144,8 @@ gitlab_rails['backup_upload_connection'] = {
   'region' => 'eu-west-1',
   'aws_access_key_id' => 'AKIAKIAKI',
   'aws_secret_access_key' => 'secret123'
-  # If using an IAM Profile, leave aws_access_key_id & aws_secret_access_key empty
-  # ie. 'aws_access_key_id' => '',
-  # 'use_iam_profile' => 'true'
+  # If using an IAM Profile, don't configure aws_access_key_id & aws_secret_access_key
+  # 'use_iam_profile' => true
 }
 gitlab_rails['backup_upload_remote_directory'] = 'my.s3.bucket'
 ```