fix xdg doc link
//!
//! Smithay currently provides two of them:
//!
-//! - The [`xdg`](xdg/index.hmtl) module provides handlers for the `xdg_shell` protocol, which is
+//! - The [`xdg`](xdg/index.html) module provides handlers for the `xdg_shell` protocol, which is
//! the current standard for desktop apps
//! - The [`legacy`](legacy/index.html) module provides handlers for the `wl_shell` protocol, which
//! is now deprecated. You only need it if you want to support apps predating `xdg_shell`.
