docsite/build-site.py should use full path to index.html

Before
--------
    foo@bar:~/ansible/mine/docsite$ ./build-site.py
    ~~snip~~
    build succeeded, 1 warning.
    gvfs-open: ~/ansible/mine/docsite/index.html: error opening location: Error when getting information for file '~ansible/mine/docsite/index.html': No such file or directory
    foo@bar:~/ansible/mine/docsite$

After
--------
    Webpage opens in browser
    Also tested "make webdocs" from top level


Also fixed minor typo in comment. Apologies if you prefer that in a separate change, let me know, I thought this would be pragmatic.