Bug#976301: Fix invalid `changelog` format example

[Guillem Jover <guillem@debian.org>]

On Mon, 2021-01-18 at 18:25:55 -0700, Sean Whitton wrote:
> On Thu 03 Dec 2020 at 05:08AM +03, Anatoli Babenia wrote:
> > diff --git a/policy/ch-source.rst b/policy/ch-source.rst
> > index edae8c1..1265c5e 100644
> > --- a/policy/ch-source.rst
> > +++ b/policy/ch-source.rst
> > @@ -126,7 +126,7 @@ That format is a series of entries like this:
> >       [blank line(s), included in output of dpkg-parsechangelog]
> >       * even more change details
> >       [optional blank line(s), stripped]
> >     [----]{+[space]--+} maintainer name <email address>[two [-spaces]
> > date-]{+spaces]date+}
> 
> I do not believe that what is already there is invalid or unclear.
> 
> Please explain your motivations.

Oh, but it does look invalid. :) Notice that the trailing maintainer
line starts at the same column as the header line, but the trailer must
be indented by one space. Also the «two spaces» are then followed by
two explicit spaces, which seems confusing.

I guess Anatoli proposed to make the whitespace requirements explicit
to avoid this kind of subtle problems in the future.

Thanks,
Guillem