[fix] Fix dead link to the KEYS file in the release process

[RobertIndie]

Motivation

This PR is to fix a dead link issue discovered by Max Xu:
https://lists.apache.org/thread/5wgxr443w2j86xcydwky3b7xjb719rtk

The link to the KEYS file is a dead link.

Modifications

• Replace the link with the correct one: https://downloads.apache.org/pulsar/KEYS

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / GoDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[maxsxu]

Thanks!