Fix typo on per-request-configuration.asciidoc

[Psyker]

Hello, i just noticed a tiny typo in this page : https://www.elastic.co/guide/en/elasticsearch/client/php-api/current/per_request_configuration.html

Thanks <3 !

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[ezimuel]

Thanks @Psyker