update example runinput.yml file from docs

[orbeckst]

@EliasCoba reports that the runinput.yml from https://mdpow.readthedocs.io/en/latest/init.html, in the section "2.3. Tutorial: Using the mdpow scripts to compute log POW of benzene", there is an hyperlink to a benzene.yml file using watermodel keyword. — #112 (comment)

TODO: replace watermodel with solventmodel

[orbeckst]

Instead of shipping the yml file with the docs, we should link it to the raw file on GitHub under the appropriate release tag, eg https://github.com/Becksteinlab/MDPOW/raw/release-0.8.0/doc/examples/benzene.yml (and the file in doc/example should be updated to contain everything that https://github.com/Becksteinlab/MDPOW/blob/release-0.8.0/mdpow/templates/runinput.yml has)