nvim: after/ftplugin/markdown: Set conceallevel to 2

neovim 0.10 introduces experimental support for hyperlinks using
OSC 8 sequence. By default, this is used in Markdown for links of
the form [example](https://example.com). If terminal supports the
OSC 8 escape sequence, then the text example in the neovim buffer
will contain a hyperlink that can be clicked and it will open in
web browser.

This means markdown files with conceallevel=2 look much closer to
the rendered output.

nvim/.config/nvim/after/ftplugin/markdown.lua

@@ -1,4 +1,5 @@
-vim.bo.textwidth = 0
+vim.bo.textwidth    = 0
+vim.wo.conceallevel = 2
 
 vim.api.nvim_create_autocmd({ "BufWritePost" }, {
   buffer  = 0,