sh4: fix broken link to documentation

The page that was previously linked in the source code and the README file is
no longer available so it now returns a 404 error message.

This puts a previous snapshot from archive.org instead.

Signed-off-by: Reda Sallahi <fullmanet@gmail.com>
Acked-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/target-sh4/README.sh4 b/target-sh4/README.sh4
index e578830..ece0464 100644
--- a/target-sh4/README.sh4
+++ b/target-sh4/README.sh4
@@ -25,7 +25,7 @@
 
 The primary model being worked on is the soft MMU target to be able to
 emulate the Shix 2.0 board by Alexis Polti, described at
-http://perso.enst.fr/~polti/realisations/shix20/
+https://web.archive.org/web/20070917001736/http://perso.enst.fr/~polti/realisations/shix20/
 
 Ultimately, qemu will be coupled with a system C or a verilog
 simulator to simulate the whole board functionalities.