Change `cmd` to `cmd_builder` in README.md

[JulianGoede]

The description in https://github.com/lspcontainers/lspcontainers.nvim#additional-languages
describes how to startup the language server using a custom command.
The actual "lspcontainers" keyword for this is cmd_builder instead of cmd:

cmd = lspcontainers.command('html', {
	image = "lspcontainers/html-language-server:1.4.0",
	cmd_builder = function (runtime, volume, image)
      return {
        runtime,
        "container",
        "run",
        "--interactive",
        "--rm",
        "--volume",
        volume,
        image
      }
    end,
  }),