doc: update neovim statusline example

Author: stevearc

README.md

@@ -181,8 +181,7 @@ the following function:
 
 ```lua
 local function arduino_status()
-  local ft = vim.api.nvim_buf_get_option(0, "ft")
-  if ft ~= "arduino" then
+  if vim.bo.filetype ~= "arduino" then
     return ""
   end
   local port = vim.fn["arduino#GetPort"]()