runtime: fix outdated g.m comment in traceback.go

Change-Id: Idfd02781a9ee550da240ef53898943e8cd37dd64
Reviewed-on: https://go-review.googlesource.com/c/go/+/418574


Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>