internal/number: fix grouping placement bug

Change-Id: I63a802fbfafe6b758e1a173bf3753e9084b31326
Reviewed-on: https://go-review.googlesource.com/46250
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed