docs: use opts for lazy.nvim sample snippet (#25)

README.md

@@ -78,7 +78,7 @@ auto_dark_mode.setup({
 ```lua
 return {
   "f-person/auto-dark-mode.nvim",
-  config = {
+  opts = {
     update_interval = 1000,
     set_dark_mode = function()
       vim.api.nvim_set_option("background", "dark")