go.dev/_content: increase HaTS download survey sample
go.dev/_content: increase HaTS download survey sample

The HaTS survey for go downloads is not yielding enough
responses. Increased the prompt rate to 20%.

Change-Id: If944cee616c86167bed45e56e558716260bd7cb5
Reviewed-on: https://go-review.googlesource.com/c/website/+/360718
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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