unicode/rangetable: updated comment

left-over from when Merge was designed to be in core unicode.

Change-Id: Id349c7e3167620e2d4eafadc1f8cda3faf4a3cf5
Reviewed-on: https://go-review.googlesource.com/11638
Reviewed-by: Nigel Tao <nigeltao@golang.org>
1 file changed