Batch path should start with `api/` not `/api/`

[chrisspiegl]

Based on docs:

The relative path of api endpoint. Must start with api/
Source: https://developers.mailerlite.com/docs/batching.html#request-body

But the example shows it with /api/

    {
      method: "GET",
      path:   "/api/forms/popup",   // <-- incorrect and potentially confusing
      body:   {
        filter: {
          name: "test popup"
        }
      }

https://github.com/mailerlite/mailerlite-nodejs/blame/4dbc1670ab289fdc8b568da08765afe3d03efead/src/modules/batches/README.md#L30C7-L30C34