cli: add missing line in --plugin-path help

cli: add missing line in --plugin-path help

When doing "babeltrace2 convert --help", a line is missing from the
--plugin-path option help.  This patch adds it, copied from the help of
the other commands.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I823385cae29afa2bfa0650d4c5dd0142dd4f9cea
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1549
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>