Documentation: translations: update linux cross reference link

The old link to source code cross reference does not work now. Though
the link has updated by commit 1d12554f0a
("Documentation: HOWTO: update code cross reference link"), there are
few obsolete links yet. This commit update them.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-docs.txt

@@ -631,7 +631,7 @@
        between two versions of a file".
 
      * Name: "Cross-Referencing Linux"
-       URL: http://lxr.linux.no/source/
+       URL: http://lxr.free-electrons.com/
        Keywords: Browsing source code.
        Description: Another web-based Linux kernel source code browser.
        Lots of cross references to variables and functions. You can see