[pgsql-www] Beta testing page refers to beta2, not beta3

https://www.postgresql.org/developer/beta/ indicates that the current PG 10 beta is beta2; however, beta3 was released on August 10.

Links to update:

I imagine there'll be a beta4, but it's not clear when, and several issues were fixed in beta3. Most people will notice on the front page, but people can still visit the testing page direct from a search.

Best regards,