Fix: sink.text.pretty: print_discarded_elements_msg(): add missing space

Fix: sink.text.pretty: print_discarded_elements_msg(): add missing space

Before:

    WARNING: Tracer discarded 17 events(unknown time range) in trace
    "ev-disc-no-ts-begin-end" (no UUID) within stream
    "/path/to/ev-disc-no-ts-begin-end/stream" (stream class ID: 0,
    stream ID: 0).

After:

    WARNING: Tracer discarded 17 events (unknown time range) in trace
    "ev-disc-no-ts-begin-end" (no UUID) within stream
    "/path/to/ev-disc-no-ts-begin-end/stream" (stream class ID: 0,
    stream ID: 0).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3f93f96479b2249c3e16f68e53533c48cf02a679
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1367
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins