Adding ignore for link validation logic to the precommit dashboard

This is breaking the precommit, so I am adding an ignore witha comment linking to this JIRA for in the Rakefile. Please remove it when the precommit dashboard is working again. Or consider making this precommit more intelligent, to only test newly added links. Or have some other intelligent way to not cause time churn on the PR due to a website going down, unrelated to the website change.

/usr/local/google/home/ajamato/go/src/github.com/apache/beam/website/Rakefile

Imported from Jira [BEAM-6455](https://issues.apache.org/jira/browse/BEAM-6455). Original Jira may contain additional context.
Reported by: ajamato@google.com.