docs(README): incorrect setup table (#406)

Author: jamestrew

README.md

@@ -229,7 +229,7 @@ local fb_actions = require "telescope".extensions.file_browser.actions
 
 require("telescope").setup {
   defaults = { --[[ your defaults]] },
-  pickers = {
+  extensions = {
     file_browser = {
       mappings = {
         ["i"] = {