time: update comment to reflect correct file path.

Update cross-reference in the comment for runtimeTimer to point to the
new go file instead of the old .h file.

Change-Id: Iddb3614c41e1989096d6caf77d6c0d5781005181
Reviewed-on: https://go-review.googlesource.com/30157Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/sleep.go

@@ -12,7 +12,7 @@ func Sleep(d Duration)
 func runtimeNano() int64
 
 // Interface to timers implemented in package runtime.
-// Must be in sync with ../runtime/runtime.h:/^struct.Timer$
+// Must be in sync with ../runtime/time.go:/^type timer
 type runtimeTimer struct {
 	i      int
 	when   int64