Update Document for index template

[vampire-yuta]

Description

When I use this example codes, Output 'Bad Requets' messages.
here

error message

│ Error: elastic: Error 400 (Bad Request): [2:15] [index_template] template doesn't support values of type: VALUE_STRING [type=x_content_parse_exception]
│
│   with module.template.opensearch_index_template.template_1,
│   on ../../modules/template/main.tf line 76, in resource "opensearch_index_template" "template_1":
│   76: resource "opensearch_index_template" "template_1" {

It is already fixed example code
I execute command tfplugindocs generate.

Please check this PR 🙏

Issues Resolved

N/A

[prudhvigodithi]

Hey @vampire-yuta, can you please rebase from main and fix the failing tests?
Thanks @bbarani @rblcoder