cmd/compile/internal/ir: fix up stale comment
cmd/compile/internal/ir: fix up stale comment

Fix a small stale comment in FinishCaptureNames (refers to old code
structure before the big refactoring).

Change-Id: I2dfb84ce238f919f6e17061439a8bd9b09459dae
Reviewed-on: https://go-review.googlesource.com/c/go/+/296829
Trust: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Run-TryBot: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

nawawi/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go