runtime: update runtime2.go itab comments about sync struct

`cmd/compile/internal/gc/reflect.go:/^func.dumptypestructs` was modified many times, now is  `cmd/compile/internal/gc/reflect.go:/^func.dumptabs`

Change-Id: Ie949a5bee7878c998591468a04f67a8a70c61da7
GitHub-Last-Rev: 9ecc26985ef18c8e870649b46419db0a9c72054f
GitHub-Pull-Request: golang/go#34489
Reviewed-on: https://go-review.googlesource.com/c/go/+/197037Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/runtime2.go

@@ -726,7 +726,7 @@ type funcinl struct {
 // layout of Itab known to compilers
 // allocated in non-garbage-collected memory
 // Needs to be in sync with
-// ../cmd/compile/internal/gc/reflect.go:/^func.dumptypestructs.
+// ../cmd/compile/internal/gc/reflect.go:/^func.dumptabs.
 type itab struct {
 	inter *interfacetype
 	_type *_type