Update target.md (hashicorp#349)

Update docs for Target resource removing `scope_id` from `boundary_host` resources to prevent error

```
╷
│ Error: Unsupported argument
│
│   on main.tf line 79, in resource "boundary_host" "foo":
│   79:   scope_id        = boundary_scope.project.id
│
│ An argument named "scope_id" is not expected here.
```

docs/resources/target.md

@@ -61,15 +61,13 @@ resource "boundary_host" "foo" {
   type            = "static"
   name            = "foo"
   host_catalog_id = boundary_host_catalog.foo.id
-  scope_id        = boundary_scope.project.id
   address         = "10.0.0.1"
 }
 
 resource "boundary_host" "bar" {
   type            = "static"
   name            = "bar"
   host_catalog_id = boundary_host_catalog.foo.id
-  scope_id        = boundary_scope.project.id
   address         = "10.0.0.1"
 }