docs: update quick start example (#2540)

update sort configuration key

Author: dvdksn

README.md

@@ -61,7 +61,9 @@ require("nvim-tree").setup()
 
 -- OR setup with some options
 require("nvim-tree").setup({
-  sort_by = "case_sensitive",
+  sort = {
+    sorter = "case_sensitive",
+  },
   view = {
     width = 30,
   },