go/build/constraint: fix splitPlusBuild func doc comment
go/build/constraint: fix splitPlusBuild func doc comment

Noticed while reading the code of the new package; likely a copy-paste
from the splitGoBuild function, which is almost identical.

Change-Id: I869272123708d25d237a4f0445f8e853865747f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/295469
Trust: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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