nvim: Fix spell dictionary not able to be found

Added the previously important 'site' directory to the runtime path of
neovim so it stops complaining about not finding spell dictionaries on
starting up with a spell-enabled file.

nvim/.config/nvim/init.lua

@@ -54,3 +54,5 @@ require("lazy").setup("plugins")
 require('look')
 require('maps')
 
+-- to include e.g. the spell dictionaries for vim
+vim.opt.rtp:append(vim.fn.stdpath("data") .. "/site")