slices: fix incorrect comment in slices.Insert function documentation

slices: fix incorrect comment in slices.Insert function documentation

The comment previously stated that r[i+len(v)] would equal the value
originally at r[i], but it should be the value originally at s[i].

Change-Id: I635ddbdd5dc4da9c06ed426d5542bf969b7fe6dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/722680
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>