update README.md.

README.md

 This is a hook script of [dehydrated](https://github.com/lukas2511/dehydrated) (former name was letsencrypt.sh) for Zope backends.
 
 First, you need to prepare the target Zope folder beforehand so that URL like http://example.com/.well-known/acme-challenge/xxx works.
-For example, you can create ".well-known/acme-challenge" in skin folder.
+For example, you can create the target folder in "portal_skins/custom/.well-known/acme-challenge".
 
 Next, you need to provide Zope's username and password in ~/.netrc like :