Clarify that changelogs should use the plural form for a "Breaking Change" Header

[majormoses]

In sensu-plugins/sensu-plugins-telegram#16 there was made aware some confusing documentation that refers to it in its singular form but in reality it should always be in plural form so it remains consistent.

Before we can update the docs to be more clear we need to check if anywhere links to that header as we don't want to give dead links.