Updates to README

[dtmarc]

In the README, I believe there are several typos that may cause confusion for the installation.

In the Development Installation there, item #7 currently reads:

cd /usr/lib/ckan/dpplus_env/src/datapusher-plus/datapusher
cp dot-env.template .env
# configure your installation as required
nano .env

I believe the reference to dpplus_env should be dpplus_venv, correct? This would require the README to correctly display the following:

cd /usr/lib/ckan/dpplus_venv/src/datapusher-plus/datapusher
cp dot-env.template .env
# configure your installation as required
nano .env