Update the command for creating Elasticsearch index mappings in tutorial-1

[vpoiesz]

Elasticsearch 6.0+ REST API now enforces strict content-type checking, so the command now includes the Content-Type header as application/json. For more info, see https://stackoverflow.com/questions/47544966/elasticsearch-content-type-header-application-x-www-form-urlencoded-is-not-s

[metadaddy]

Thanks, @vpoiesz!