Update DEBIAN_HOW_TO.md

[dhruvbird]

Why does this diff add a dot at the end of the clone command?

[harobed]

I've appended dot, to clone source code to current directory (/usr/local/lib/bosh).

When I see :

cd /usr/local/lib/bosh
git pull

then I understand that source code must be installed in /usr/local/lib/bosh

Without dot, source code is installed in /usr/local/lib/bosh/node-xmpp-bosh