Fix watcher config example in README

[samgaw]

Example config errors out due to the way opts is parsed to LightningCSS.run/3:

** (FunctionClauseError) no function clause matching in Keyword.get/3
    (elixir 1.15.7) Keyword.get({:watch, true}, :watch, false)
    (lightning_css 0.4.0) lib/lightning_css.ex:44: LightningCSS.run/3
    (phoenix 1.7.10) lib/phoenix/endpoint/watcher.ex:19: Phoenix.Endpoint.Watcher.watch/2
    (elixir 1.15.7) lib/task/supervised.ex:101: Task.Supervised.invoke_mfa/2
Function: &Phoenix.Endpoint.Watcher.watch/2
    Args: ["css", {LightningCSS, :install_and_run, [:default, [], {:watch, true}]}]

Explicitly passing opts as a list fixes the issue.