Bug#976301: Fix invalid `changelog` format example

[Anatoli Babenia]

It is hard to spot the space in front of the string, and much harder
to understand that it is significant. That's why I still think that
applying my patch as-is is a better choice. In committed change
https://salsa.debian.org/dbnpolicy/policy/-/commit/69933a335bce539ec8e75f3b5625dc69509d9886
the test that explains blank lines goes as replacement for those blank
like, but the text that explains two space pattern is an inline
comment. In my patch user needs to replace every placeholder with the
content that is described within.