Update getting_started.md - replace deprecated API method

[924312]

PR Summary

Not super familiar with Neovim but this is deprecated

vim.api.nvim_buf_set_option(bufnr, 'omnifunc', 'v:lua.vim.lsp.omnifunc')

and I believe this is the fix

vim.api.nvim_set_option_value("omnifunc", "v:lua.vim.lsp.omnifunc", { buf = bufnr })

PR Context