runtime: update link to WriteHeapDump format

The new link is https://golang.org/s/go15heapdump.

Change-Id: Ifcaf8572bfe815ffaa78442a1991f6e20e990a50
Reviewed-on: https://go-review.googlesource.com/20740Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/debug/garbage.go

@@ -155,7 +155,7 @@ func SetPanicOnFault(enabled bool) bool {
 
 // WriteHeapDump writes a description of the heap and the objects in
 // it to the given file descriptor.
-// The heap dump format is defined at https://golang.org/s/go13heapdump.
+// The heap dump format is defined at https://golang.org/s/go15heapdump.
 func WriteHeapDump(fd uintptr)
 
 // SetTraceback sets the amount of detail printed by the runtime in

src/runtime/heapdump.go

@@ -7,7 +7,7 @@
 // finalizers, etc.) to a file.
 
 // The format of the dumped file is described at
-// https://golang.org/s/go14heapdump.
+// https://golang.org/s/go15heapdump.
 
 package runtime