os: add period at end of O_EXCL comment
os: add period at end of O_EXCL comment

Change-Id: I9d723e49bf2cabf97a09e29e46fd2c426845fd1d
Reviewed-on: https://go-review.googlesource.com/71470
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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