fix: rename vim.highlight.on_yank to vim.hl.on_yank

The functions of vim.highlight were renamed to vim.hl on commit
18b43c331d8a0ed87d7cbefe2a18543b8e4ad360 of neovim, which was applied
with the release of nvim version 0.11.

Now, the use of vim.highlight is deprecated, and instead, one should
use vim.hl functions.
In practice, vim.highlight is still working, however, asking for help
for vim.highlight.on_yank fails (E149), while asking for help for
vim.hl.on_yank works as expected. So, by updating the used function, a
new user will have easier time looking getting the relevant help.

init.lua

@@ -210,7 +210,7 @@ vim.api.nvim_create_autocmd('TextYankPost', {
   desc = 'Highlight when yanking (copying) text',
   group = vim.api.nvim_create_augroup('kickstart-highlight-yank', { clear = true }),
   callback = function()
-    vim.highlight.on_yank()
+    vim.hl.on_yank()
   end,
 })