cmd/compile: fix comment to reference runtime.countrunes
cmd/compile: fix comment to reference runtime.countrunes

Updates #24923

Change-Id: Ie5a1b54b023381b58df618080f3d742a50d46d8b
Reviewed-on: https://go-review.googlesource.com/115836
Reviewed-by: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Run-TryBot: Martin Möhrmann <9c82b83f348009b39856043bb9ed3ef41e171aeb@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,golang/go,josharian/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,josharian/go,CAFxX/go,golang/go,akutz/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,golang/go,josharian/go,akutz/go,akutz/go,nawawi/go,josharian/go,golang/go,josharian/go,nawawi/go,nawawi/go,akutz/go,akutz/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,josharian/go,cloudflare/go,josharian/go,golang/go,josharian/go,cloudflare/go