doc: interpret-trailers: use “metadata” in Name as well

doc: interpret-trailers: use “metadata” in Name as well

We now since the previous commit introduce the format as “trailer
metadata”. We can replace “structured information” with “metadata”
in the “Name” section to be consistent.

While “structured information” does emphasize that the data is not
loosely structured, we also say that this command adds to or parses
this format. I don’t think that we need to emphasize that it is
structured since clearly there is some structure there.

Both “metadata” and “structured information” can convey the same
information. But “metadata” is shorter and easier to deploy since
it’s just one word.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>