doc/go1.12: remove known issue note
doc/go1.12: remove known issue note

A workaround has been submitted.

Updates #27993

Change-Id: Ife6443c32673b38000b90dd2efb2985db37ab773
Reviewed-on: https://go-review.googlesource.com/c/156318
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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