[dev.simd] simd: fix doc comments in slicepart_128.go

In the Int<->Uint swap induced by the change to bit-punning
methods, some comments were missed.

Change-Id: I7fb6a10195f01816e310ac9f1e649bbde18450ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/783920
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>