Fix cargo doc on older rustc

Apparently at least rustc 1.48 doesn't support `Self` in doc links,
so we make it explicit.

Author: TheBlueMatt

lightning-background-processor/src/lib.rs

@@ -153,7 +153,7 @@ impl BackgroundProcessor {
 	/// functionality implemented by other handlers.
 	/// * [`NetGraphMsgHandler`] if given will update the [`NetworkGraph`] based on payment failures.
 	///
-	/// [top-level documentation]: Self
+	/// [top-level documentation]: BackgroundProcessor
 	/// [`join`]: Self::join
 	/// [`stop`]: Self::stop
 	/// [`ChannelManager`]: lightning::ln::channelmanager::ChannelManager