Documentation: HOWTO: update code cross reference link

The source code cross reference link does not work.
Update the link to a site that works.
Signed-off-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/HOWTO

@@ -209,7 +209,7 @@ tools.  One such tool that is particularly recommended is the Linux
 Cross-Reference project, which is able to present source code in a
 self-referential, indexed webpage format. An excellent up-to-date
 repository of the kernel code may be found at:
-	http://lxr.linux.no/+trees
+	http://lxr.free-electrons.com/
 
 
 The development process