Add section on changing route prefix.

[andyrue]

Thought I would suggest adding this to the docs to make it a little clearer how to change the prefix.

[janko]

I understand that changing the path prefix is common, but in rodauth-rails docs I'm trying to avoid repeating official Rodauth documentation, and have it focused only on the new behaviour rodauth-rails adds. In this case, I'm not clear how this helps compared to the official docs:

[andyrue]

Oh shoot, you're right. My apologies. I was on the official docs originally, but I guess I only noticed the prefix "/auth" part and not the additional block. Sorry!