Fix documentation error (#1372)

Author: BubuDavid

README.markdown

@@ -170,7 +170,7 @@ autocmd BufEnter * if winnr('$') == 1 && exists('b:NERDTree') && b:NERDTree.isTa
 
 ```vim
 " If another buffer tries to replace NERDTree, put it in the other window, and bring back NERDTree.
-autocmd BufEnter * if bufname('#') =~ 'NERD_tree_\d\+' && bufname('%') !~ 'NERD_tree_\d\+' && winnr('$') > 1 |
+autocmd BufEnter * if winnr() == winnr('h') && bufname('#') =~ 'NERD_tree_\d\+' && bufname('%') !~ 'NERD_tree_\d\+' && winnr('$') > 1 |
     \ let buf=bufnr() | buffer# | execute "normal! \<C-W>w" | execute 'buffer'.buf | endif
 ```