Update lb_target_group docs

- hashicorp#19602 Issue solved

Description
[This PR](terraform-aws-modules/terraform-aws-alb#160) is merged. But not apply in terraform docs in site
default valud of proxy_protocol_v2 is changed from null to false
The changes were not reflected in the document.

website/docs/r/lb_target_group.html.markdown

@@ -68,7 +68,7 @@ The following arguments are supported:
 * `slow_start` - (Optional) The amount time for targets to warm up before the load balancer sends them a full share of requests. The range is 30-900 seconds or 0 to disable. The default value is 0 seconds.
 * `load_balancing_algorithm_type` - (Optional) Determines how the load balancer selects targets when routing requests. Only applicable for Application Load Balancer Target Groups. The value is `round_robin` or `least_outstanding_requests`. The default is `round_robin`.
 * `lambda_multi_value_headers_enabled` - (Optional) Boolean whether the request and response headers exchanged between the load balancer and the Lambda function include arrays of values or strings. Only applies when `target_type` is `lambda`.
-* `proxy_protocol_v2` - (Optional) Boolean to enable / disable support for proxy protocol v2 on Network Load Balancers. See [doc](https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#proxy-protocol) for more information.
+* `proxy_protocol_v2` - (Optional) Boolean to enable / disable support for proxy protocol v2 on Network Load Balancers. Default is `false`. See [doc](https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#proxy-protocol) for more information.
 * `stickiness` - (Optional, Maximum of 1) A Stickiness block. Stickiness blocks are documented below.
 * `health_check` - (Optional, Maximum of 1) A Health Check block. Health Check blocks are documented below.
 * `target_type` - (Optional, Forces new resource) The type of target that you must specify when registering targets with this target group.