update doc g:nvim_tree_icons to match defaults

README.md

@@ -64,8 +64,8 @@ let g:nvim_tree_show_icons = {
 " default will show icon by default if no icon is provided
 " default shows no icon by default
 let g:nvim_tree_icons = {
-    \ 'default': '',
-    \ 'symlink': '',
+    \ 'default': "",
+    \ 'symlink': "",
     \ 'git': {
     \   'unstaged': "✗",
     \   'staged': "✓",

doc/nvim-tree-lua.txt

@@ -555,8 +555,8 @@ You can set icons for:
 - Symlinks. If an icon is not provided, the `default` icon is used.
 >
   let g:nvim_tree_icons = {
-      \ 'default':        '',
-      \ 'symlink':        '',
+      \ 'default':        "",
+      \ 'symlink':        "",
       \ 'git': {
       \   'unstaged':     "✗",
       \   'staged':       "✓",