design/43651-type-parameters: use ~ in SliceConstraint
design/43651-type-parameters: use ~ in SliceConstraint

Change-Id: Ifb4c3bcdc382d64b469616f311399af6fbe180f1
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/366014
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal