Mark vimscript block as "vim" (#2121)

The example configuration is Vimscript, with a heredoc containing some
Lua code. Using "lua" as a language identifier results neither the
Vimscript portion nor the Lua portion being highlighted propertly.

Mark the code block as "vim".  Neovim (with treesitter) properly
highlights the outer block as Vimscript, and the Lua heredoc as Lua
code.

README.md

@@ -29,7 +29,7 @@ Setup
 
 This example configuration uses `vim-plug` as the plugin manager and `vim-vsnip` as a snippet plugin.
 
-```lua
+```vim
 call plug#begin(s:plug_dir)
 Plug 'neovim/nvim-lspconfig'
 Plug 'hrsh7th/cmp-nvim-lsp'