Tweak outdated comment about lazy's config key usage. (nvim-lua#1250)

Remove outdated comment describing use of `config` key, replacing with
corrected `opt` key note.

Fixes nvim-lua#1249

init.lua

@@ -267,9 +267,8 @@ require('lazy').setup({
   -- which loads which-key before all the UI elements are loaded. Events can be
   -- normal autocommands events (`:help autocmd-events`).
   --
-  -- Then, because we use the `config` key, the configuration only runs
-  -- after the plugin has been loaded:
-  --  config = function() ... end
+  -- Then, because we use the `opt` key (recommended), the configuration runs
+  -- after the plugin has been loaded as `require(MODULE).setup(opts)`.
 
   { -- Useful plugin to show you pending keybinds.
     'folke/which-key.nvim',