runtime: update URL for heap dump format

I just created that redirect, so we can change
it once the wiki moves.

LGTM=bradfitz, khr
R=khr, bradfitz
CC=golang-codereviews
https://golang.org/cl/177780043

src/runtime/heapdump.c

@@ -7,7 +7,7 @@
 // finalizers, etc.) to a file.
 
 // The format of the dumped file is described at
-// http://code.google.com/p/go-wiki/wiki/heapdump14
+// http://golang.org/s/go14heapdump.
 
 #include "runtime.h"
 #include "arch_GOARCH.h"