fixed auto-mode-alist

This code:

```lisp
(add-to-list 'auto-mode-alist
 '("nginx\.conf$" . nginx-mode)
 '("/etc/nginx/.*" . nginx-mode))
```

was broken in two ways:

1. wrong regexps
2. wrong `add-to-list` syntax: the third argument is `append`, not an additional item.

Changed the auto-mode-alist part so it works now (slightly changed the actual regexps, reflected this in docs, users are likely not to notice this at all).