Fix broken link in documentation
The link to the Window struct on the [glutin index page is
broken][0]. It looks for the Window struct in the top-level module but
it's defined in the `window` module.
This change prefixes the link with the window module name.
[0]:
//! You can also produce headless [`Context`]s via the
//! [`ContextBuilder::build_headless`] function.
//!
-//! [`Window`]: struct.Window.html
+//! [`Window`]: window/struct.Window.html
//! [`Context`]: struct.Context.html
//! [`WindowedContext<T>`]: type.WindowedContext.html
//! [`RawContext<T>`]: type.RawContext.html
