Do not require trailing slash in repository url

The docs give this example:
```
repository hackage.haskell.org
url: http://hackage.haskell.org/
secure: True
root-keys:
key-threshold:
```
Note the trailing slash in `url`. In my experiments, I skipped that, and things fell over.

It’s a minor thing, but it would be nice if
```
repository hackage.haskell.org
url: http://hackage.haskell.org
secure: True
root-keys:
key-threshold:
```
would work just as well.