chore: update Vim8 txt.

Author: rzvxa

README.markdown

@@ -166,7 +166,7 @@ autocmd BufEnter * if tabpagenr('$') == 1 && winnr('$') == 1 && exists('b:NERDTr
 autocmd BufEnter * if winnr('$') == 1 && exists('b:NERDTree') && b:NERDTree.isTabTree() | call feedkeys(":quit\<CR>:\<BS>") | endif
 ```
 
-#### Vim8
+#### Vim8 or older
 
 ```vim
 " Exit Vim if NERDTree is the only window remaining in the only tab.