Fix readme examples for custom paths

I was trying to get the custom paths working using the `collection_path` attribute. This feature looked broken (#231) but I think it's just the README that is out of date. I looked at [the code](https://github.com/remiprev/her/blob/master/lib/her/model/paths.rb#L41-L86) and found that `collection_path` should only change the base path with no attributes and that `resource_path` can take attributes. 

This PR changes the README examples by changing `collection_path` to `resource_path` where required and also adds an example for changing the base path using the `collection_path` attribute.