Update section on vimscript options for vim.opt

[clason]

neovim/neovim#13479 just got merged 🎉 with the following changes:

• vim.o (set global option) was renamed to vim.go
• a new vim.o was added that behaves (essentially) like set (is expected to -- meaning there may be weird corner cases)
• vim.opt was added to allow a more "luanatic" way of setting and getting (in particular, appending) options.

The docs are quite good and explicit, so maybe it's not necessary to repeat much (or just copy the examples from the docs), but the change in vim.o meaning should be reflected.

[nanotee]

Thank you (once again!) for the reminder. Hopefully what I've added in 40d7b05 and #64 is sufficient. Very exciting feature!

[clason]

No worries! Sorry for just dropping these reminders on you rather than actually helping out with a PR...

And thanks (again!) for this excellent guide, it's really helpful!