docs: add missing AWS_IAM to auth_provider in appsync_api (hashicorp#44166)

Author: auvred

website/docs/r/appsync_api.html.markdown

@@ -131,7 +131,7 @@ The `event_config` block supports the following:
 
 The `auth_provider` block supports the following:
 
-* `auth_type` - (Required) Type of authentication provider. Valid values: `AMAZON_COGNITO_USER_POOLS`, `AWS_LAMBDA`, `OPENID_CONNECT`, `API_KEY`.
+* `auth_type` - (Required) Type of authentication provider. Valid values: `API_KEY`, `AWS_IAM`, `AMAZON_COGNITO_USER_POOLS`, `OPENID_CONNECT`, `AWS_LAMBDA`.
 * `cognito_config` - (Optional) Configuration for Cognito user pool authentication. Required when `auth_type` is `AMAZON_COGNITO_USER_POOLS`. See [Cognito Config](#cognito-config) below.
 * `lambda_authorizer_config` - (Optional) Configuration for Lambda authorization. Required when `auth_type` is `AWS_LAMBDA`. See [Lambda Authorizer Config](#lambda-authorizer-config) below.
 * `openid_connect_config` - (Optional) Configuration for OpenID Connect. Required when `auth_type` is `OPENID_CONNECT`. See [OpenID Connect Config](#openid-connect-config) below.