Convert integer to string in elasticsearch healthcheck

[penguincoder]

Humble apologies. My rigor in testing was not complete in my last elasticsearch PR. This will correct the error when running devenv up with services.elasticsearch.enable = true.

at «github:cachix/devenv/c51a56bac8853c019241fe8d821c0a0d82422835»/src/modules/services/elasticsearch.nix:187:83:

   186|         readiness_probe = {
   187|           exec.command = "${pkgs.curl}/bin/curl -f -k http://${cfg.listenAddress}:${cfg.port}";
      |                                                                                   ^
   188|           initial_delay_seconds = 15;

error: cannot coerce an integer to a string