Fixed syntax error

Author: iamWing

.vimrc

@@ -78,7 +78,7 @@ map <C-\> :NERDTreeToggle<CR>
 autocmd StdinReadPre * let s:std_in=1
 autocmd VimEnter * if argc() == 0 && ! exists("s:std_in") | NERDTree | endif
 " close vim if the only window left open is NERDTree
-autocmd bufenter * if (winnr("?") == 1 && exists("b:NERDTree")
+autocmd bufenter * if (winnr("$") == 1 && exists("b:NERDTree")
     \ && b:NERDTree.isTabTree()) | q | endif
 let NERDTreeShowHidden=1    " show hidden file by default
 let NERDTreeIgnore=['.git$[[dir]]', '.swp', '.DS_Store']