docs: Update README.md for VPC Endpoints submodule (#1216)

Update README.md

Add missed parameter for VPC Endpoint Gateway service type

Author: szhekpisov-katanox

modules/vpc-endpoints/README.md

@@ -22,6 +22,7 @@ module "endpoints" {
     dynamodb = {
       # gateway endpoint
       service         = "dynamodb"
+      service_type    = "Gateway"
       route_table_ids = ["rt-12322456", "rt-43433343", "rt-11223344"]
       tags            = { Name = "dynamodb-vpc-endpoint" }
     },