Fix warning: index.html redirects to README.html, which does not exist

[eksperimental]

I cannot tell what is the test that is causing this line, but it is really annoying, and I thought it was the change I was working on but it was not.

[lalo2302]

Could you describe the process you followed, so I can reproduce it

[eksperimental]

simply run mix test you will see it.
for example
https://travis-ci.org/elixir-lang/ex_doc/jobs/220122262#L1894