docs/ec2_client_vpn_endpoint: fix dns_servers docs (#22367)

The docs for the `dns_servers` argument of the
`aws_ec2_client_vpn_endpoint` resource is erroneous. When not specified,
the Client VPN does not automatically use the VPC DNS address. This can
be verified by checking the [AWS API docs][1]:

> If no DNS server is specified, the DNS address configured on the
> device is used for the DNS server.

[1]: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_CreateClientVpnEndpoint.html

website/docs/r/ec2_client_vpn_endpoint.html.markdown

@@ -40,7 +40,7 @@ The following arguments are supported:
 * `client_cidr_block` - (Required) The IPv4 address range, in CIDR notation, from which to assign client IP addresses. The address range cannot overlap with the local CIDR of the VPC in which the associated subnet is located, or the routes that you add manually. The address range cannot be changed after the Client VPN endpoint has been created. The CIDR block should be /22 or greater.
 * `connection_log_options` - (Required) Information about the client connection logging options.
 * `description` - (Optional) A brief description of the Client VPN endpoint.
-* `dns_servers` - (Optional) Information about the DNS servers to be used for DNS resolution. A Client VPN endpoint can have up to two DNS servers. If no DNS server is specified, the DNS address of the VPC that is to be associated with Client VPN endpoint is used as the DNS server.
+* `dns_servers` - (Optional) Information about the DNS servers to be used for DNS resolution. A Client VPN endpoint can have up to two DNS servers. If no DNS server is specified, the DNS address of the connecting device is used.
 * `server_certificate_arn` - (Required) The ARN of the ACM server certificate.
 * `split_tunnel` - (Optional) Indicates whether split-tunnel is enabled on VPN endpoint. Default value is `false`.
 * `self_service_portal` - (Optional) Specify whether to enable the self-service portal for the Client VPN endpoint. Values can be `enabled` or `disabled`. Default value is `disabled`.