cairo: Fix README link to docs
The previous link was a 404
@@ -4,7 +4,7 @@ Cairo bindings for Rust.
- [Gtk-rs project site](http://gtk-rs.org/)
-- [Online documentation](http://gtk-rs.org/docs/)
+- [Online documentation](https://gtk-rs.org/docs-src/)
- [Readme](https://github.com/gtk-rs/gtk/blob/master/README.md) in our
[main repo](https://github.com/gtk-rs/gtk)
