doc/go1.15: exclude spaces from <code> block
doc/go1.15: exclude spaces from <code> block

Per the note at the top of go1.15.html.

Updates #37419.

Change-Id: Ia6917347ca1e3ebe8c55f9c0ec74e49ff481a64f
Reviewed-on: https://go-review.googlesource.com/c/go/+/236719
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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