This commit ensures that the links from the index page to the
examples pages of the generated HTML documentation are not broken
when the resource name includes special characters like :.

I'm aware this pull request needs tests to be accepted; however I was not sure how to write the required tests.
I will explain how to reproduce the bug solved by this PR here and hopefully get some feedback on how to write the test (and update this PR accordingly):

1. chdir to example
2. edit spec/acceptance/orders_spec.rb, changing resource "Orders" to resource "Super::Orders"
3. run rake docs:generate
4. open doc/api/index.html in the browser
5. click on any example link, for instance Creating an order
6. The link will wrongly point to super::orders/creating_an_order.html, that is, it will interpret super as the URL protocol, rather than linking to an HTML file in a subfolder

My commit replaces any non-ASCII character with an underscore, which generates filesystem-acceptable filenames