Update the documentation to include description of engine_version

Author: tabito-hara

website/docs/r/opensearch_package.html.markdown

@@ -41,8 +41,9 @@ resource "aws_opensearch_package" "example" {
 This resource supports the following arguments:
 
 * `region` - (Optional) Region where this resource will be [managed](https://docs.aws.amazon.com/general/latest/gr/rande.html#regional-endpoints). Defaults to the Region set in the [provider configuration](https://registry.terraform.io/providers/hashicorp/aws/latest/docs#aws-configuration-reference).
+* `engine_version` - (Optional, Forces new resources) Engine version that the package is compatible with. This argument is required and only valid when `package_type` is `ZIP-PLUGIN`. Format: `OpenSearch_X.Y` or `Elasticsearch_X.Y`, where `X` and `Y` are the major and minor version numbers, respectively.
 * `package_name` - (Required, Forces new resource) Unique name for the package.
-* `package_type` - (Required, Forces new resource) The type of package.
+* `package_type` - (Required, Forces new resource) The type of package. Valid values are `TXT-DICTIONARY`, `ZIP-PLUGIN`, `PACKAGE-LICENSE` and `PACKAGE-CONFIG`. 
 * `package_source` - (Required, Forces new resource) Configuration block for the package source options.
 * `package_description` - (Optional, Forces new resource) Description of the package.