nixpkgs/neovim: add treesitter markdown

.config/nixpkgs/neovim/neovim.nix

@@ -68,6 +68,7 @@ let
         tree-sitter-javascript
         tree-sitter-json
         tree-sitter-lua
+        tree-sitter-markdown
         tree-sitter-nix
         tree-sitter-ruby
         tree-sitter-tsx