Fix broker robots.txt to disallow crawling

From comment:11:ticket:28848 and https://github.com/ahf/snowflake-notes/blob/fb4304a7df08c6ddeeb103f38fc9103721a20cd9/Broker.markdown#the-robotstxt-handler:

• Was the question about crawling ever answered? I can't think of a very good reason not to allow it. Even if censors were crawling the web for Snowflake brokers, they could get this information much more easily just from the source code.

I believe the intention behind the robots.txt handler is to prevent search engines from indexing any pages on the site, because there's no permanent information there, not for any security or anti-enumeration reason.

ahf points out that the current robots.txt achieves the opposite: it allows crawling of all pages by anyone. Instead of

User-agent: *
Disallow:

it should be

User-agent: *
Disallow: /