Fix esy sandbox setup

[AriaFallah]

This wasn't working for me, and I eventually found this #1222 (comment)

{
  "ocaml.sandbox": {
    "kind": "esy",
    "root": "${firstWorkspaceFolder}"
  }
}

Is the correct way to do this by default

[smorimoto]

@AriaFallah Could you add a change log entry?

[mnxn]

That section of the README is just an example configuration. The old version wasn't incorrect, it just has the name of the folder in the variable, which means the example only works in workspaces with the vscode-ocaml-platform folder. If you use the sandbox selector UI or commands in VS Code, it should automatically change absolute paths to use the correct variable for your environment.

That being said, changing the example is a good idea if it's misleading users this way.
The README should probably document both ways (${workspaceFolder:folderName} and ${firstWorkspaceFolder}) in the future though.