Changelog URL in update notification mail results in 404

The changelog URL in update notification mails contains a hard-coded segment `piwik-` which results in a 404 page. It should be `matomo-`.

## Current url
> View the changelog for this version:
> https://matomo.org/changelog/piwik-3-5-1/

## Fixed url
> View the changelog for this version:
> https://matomo.org/changelog/matomo-3-5-1/