runtime/map: update comment for gc/reflect

update comment cause gc/reflect.go has been moved to reflectdata/reflect.go

In the commit (attach below), gc/reflect.go is moved to reflectdata/reflect.go
So the  comment referring gc/reflect.go should be updated to reflectdata/reflect.go

There maybe other places that refers gc/reflect.go that should be updated.
I would work around it soon.

commit:
https://github.com/golang/go/commit/de65151e507e7b3c8e46d74f223d7c562177bedc
https://github.com/golang/go/commit/e4895ab4c0eb44de6ddc5dc8d860a827b20d2781

Change-Id: Ieed5c48049ffe6889c08e164972fc7825653ac05
GitHub-Last-Rev: eec9c2328d0be40842c3994f26f26f03fa650a91
GitHub-Pull-Request: golang/go#45421
Reviewed-on: https://go-review.googlesource.com/c/go/+/307930


Reviewed-by: Keith Randall <khr@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>