doc: fix rst syntax in interfaces.rst

There are some rendering errors in https://eos-docs.web.cern.ch/diopside/manual/interfaces.html

• Put two dots before index:: so it renders properly (currently shows up as raw text)
• Code blocks seem to need bash to appear properly (currently these ones are invisible in the webpage)
  • Andreas also noticed and fixed this in 7a3a0ac9 , which I rebased on

Also fix some typos.