Update URL of repositories in README.md

We provide tools for a number of repeated tasks, such as tagging, upgrading,

knowing what to build and what to upgrade, etc. They are provided in the

`pkg-haskell-tools` package, available in sid, or at

ssh://git.debian.org/git/pkg-haskell/pkg-haskell-tools.git

(git://anonscm.debian.org/pkg-haskell/pkg-haskell-tools.git for

non-Debian-developers).

git@salsa.debian.org:haskell-team/pkg-haskell-tools.git

(git://salsa.debian.org/haskell-team/pkg-haskell-tools.git for

read-only access).

All tools are under the `dht` prefix, and it has a manpage (man dht) or at

<file:///usr/share/doc/pkg-haskell-tools/dht.html>.