doc: fix old broken link

The Camlistore code tree rearranged after the go tool came
out. (I didn't know this link was here until I saw it in
some logs.)

R=adg
CC=golang-dev
https://golang.org/cl/7374043

doc/articles/error_handling.html

@@ -137,7 +137,7 @@ messages:
 
 <p>
 (This is a slightly simplified version of some
-<a href="http://camlistore.org/code/?p=camlistore.git;a=blob;f=lib/go/camli/jsonconfig/eval.go#l68">actual code</a>
+<a href="http://golang.org/s/camjsondecode">actual code</a>
 from the <a href="http://camlistore.org">Camlistore</a> project.)
 </p>