doc/go1.13: add missing slashes
doc/go1.13: add missing slashes

This saves a redirect and makes the document more consistent.

Change-Id: Ib7f68b1967275c0c676a044314919449680297f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/191537
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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