Broken links: they are pointing to lhs instead of html

For instance https://haskell-servant.readthedocs.io/en/stable/tutorial/Client.html points to `Server.lhs` instead of `Server.html`:
```

While defining handlers that serve an API has a lot to it
```
The issue could be solved - at least in my environment - by removing `.lhs` suffix in
https://github.com/haskell-servant/servant/blob/7de93f9a51307cfdba62e3f1038f0053d6fa58fb/doc/tutorial/Client.lhs#L3