doc: interpret-trailers: don’t use heredoc in examples

doc: interpret-trailers: don’t use heredoc in examples

This file contains four instances of trailing spaces from its inception
in commit [1]. These spaces might be intentional, since a user would be
prompted with `> ` in an interactive session. On the one hand, this is a
whitespace error according to `git diff --check`; on the other hand, the
raw documentation—it makes no difference in the rendered output—is just
staying faithful to the simulation of the interactive prompt.

Let’s get rid of these whitespace errors and also make the examples more
friendly to cut-and-paste by replacing the heredocs with files which are
shown with cat(1).

[1]: dfd66ddf5a (Documentation: add documentation for 'git
    interpret-trailers', 2014-10-13)

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