docs: reformat quickstart readme example (#2258)

Author: sand4rt

README.md

@@ -79,27 +79,27 @@ require("nvim-tree").setup({
 Optionally customise your mappings, see [:help nvim-tree-mappings](doc/nvim-tree-lua.txt)
 
 ```lua
-  local function my_on_attach(bufnr)
-    local api = require "nvim-tree.api"
+local function my_on_attach(bufnr)
+  local api = require "nvim-tree.api"
 
-    local function opts(desc)
-      return { desc = "nvim-tree: " .. desc, buffer = bufnr, noremap = true, silent = true, nowait = true }
-    end
+  local function opts(desc)
+    return { desc = "nvim-tree: " .. desc, buffer = bufnr, noremap = true, silent = true, nowait = true }
+  end
 
-    -- default mappings
-    api.config.mappings.default_on_attach(bufnr)
+  -- default mappings
+  api.config.mappings.default_on_attach(bufnr)
 
-    -- custom mappings
-    vim.keymap.set('n', '<C-t>', api.tree.change_root_to_parent,        opts('Up'))
-    vim.keymap.set('n', '?',     api.tree.toggle_help,                  opts('Help'))
-  end
+  -- custom mappings
+  vim.keymap.set('n', '<C-t>', api.tree.change_root_to_parent,        opts('Up'))
+  vim.keymap.set('n', '?',     api.tree.toggle_help,                  opts('Help'))
+end
 
-  -- pass to setup along with your other options
-  require("nvim-tree").setup {
-    ---
-    on_attach = my_on_attach,
-    ---
-  }
+-- pass to setup along with your other options
+require("nvim-tree").setup {
+  ---
+  on_attach = my_on_attach,
+  ---
+}
 ```
 
 Open the tree:  `:NvimTreeOpen`