update sample and add error to reflect the default basedir: .*/fdroid/repo

update sample and add error to reflect the default basedir: .*/fdroid/repo

For user-generated repos, the default path/URL is .*/fdroid/repo, with
fdroid/ as the root where the 'fdroid' tool operates.  This makes for a URL
that is quite unique and easily matched automatically using patterns, like
in fdroidclient.

For those who don't like the standard, they can override the errror from
config.py using nonstandardwebroot = True