Documentation: Updated broken link in HOWTO

Updated broken "The Perfect Patch" link in HOWTO to a copy at kerneltrap.org.
Signed-off-by: Ben Minerds <puzzleduck@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/HOWTO

@@ -112,7 +112,7 @@ required reading:
 
     Other excellent descriptions of how to create patches properly are:
 	"The Perfect Patch"
-		http://userweb.kernel.org/~akpm/stuff/tpp.txt
+		http://kerneltrap.org/node/3737
 	"Linux kernel patch submission format"
 		http://linux.yyz.us/patch-format.html