fix commentstring for elixir files (#1321)

ftplugin/elixir.lua

@@ -1,4 +1,5 @@
 require("lsp").setup "elixir"
+vim.api.nvim_buf_set_option(0, "commentstring", "# %s")
 
 -- TODO: do we need this?
 -- needed for the LSP to recognize elixir files (alternatively just use elixir-editors/vim-elixir)