os: fix grammar nit
os: fix grammar nit

There should not be a comma before "and" in the original text,
because what follows is not a complete sentence. Rewrite.

Change-Id: Ie99f204cc87e911fb46149e2eb65e132fa1eb63a
Reviewed-on: https://go-review.googlesource.com/87020
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

akutz/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,akutz/go,josharian/go,nawawi/go,CAFxX/go,CAFxX/go,Cofyc/go,josharian/go,cloudflare/go,christopher-henderson/Go,josharian/go,golang/go,nawawi/go,Cofyc/go,golang/go,nawawi/go,akutz/go,codestation/go,cloudflare/go,golang/go,josharian/go,cloudflare/go,nawawi/go,Cofyc/go,CAFxX/go,Cofyc/go,josharian/go,Cofyc/go,codestation/go,golang/go,CAFxX/go,josharian/go,codestation/go,codestation/go,golang/go,Cofyc/go,cloudflare/go,cloudflare/go,josharian/go,codestation/go,akutz/go,akutz/go,akutz/go,codestation/go,golang/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,codestation/go,CAFxX/go,Cofyc/go,akutz/go,cloudflare/go,nawawi/go,Cofyc/go,codestation/go,golang/go,josharian/go,akutz/go