Index template comment doesn't work

[kesha]

Logstash: 6.5.4

Index template:

{
  "_rev": "$Rev: 902 $",
  "_comment": "Copyright © 2018, CCG, All rights reserved. ",

  "order": 0,
  "index_patterns": ["ossec-alerts-*"],
    "settings": {
     "index" : {
            "number_of_shards" : 3,
            "number_of_replicas" : 2
     },
     "refresh_interval": "5s",
     "index.mapping.ignore_malformed" : true,
     "index.codec" : "best_compression"
  },
  "mappings": {

If I keep the first two line comments inside the template, logstash will report 400 error can not get access to elasticsearch.

[ERROR][logstash.outputs.elasticsearch] Failed to install template. {:message=>"Got response code '400' contacting Elasticsearch at URL 'http://127.0.0.1:9200/_template/elastiflow'", :class=>"LogStash::Outputs::ElasticSearch::HttpClient::Pool::BadResponseCodeError", 

According to the official document, it claims that we can use /**/ to comment info in template

Index templates provide C-style /* */ block comments. Comments are allowed everywhere in the JSON document except before the initial opening curly bracket.

So I added the /* */ pair to the first two lines, but there's still the error, it can not parse the template.
How can I add comments in the index template?