sink.text.details: don't write `:` after `Packet beginning` without ctx

sink.text.details: don't write `:` after `Packet beginning` without ctx

This patch fixes packet beginning lines which look like:

    ...
        Stream (ID 0, Class ID 0)

    {Trace 0, Stream class ID 0, Stream ID 0}
    Packet beginning:

    {Trace 0, Stream class ID 0, Stream ID 0}
    ...

so that they become:

    ...
        Stream (ID 0, Class ID 0)

    {Trace 0, Stream class ID 0, Stream ID 0}
    Packet beginning

    {Trace 0, Stream class ID 0, Stream ID 0}
    ...

In other words, the `:` character following `Packet beginning` is
useless as there's no (packet context) data following.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4c75bc487c10fa2a4b39708d09d4089887ceabbd
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2074
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>