jack_load_test: fix parameter help

[fps]

The help text of jack_load_test says:

        -t, --timeout         Wait timeout in seconds

But the time is interpreted as microseconds:

See:

https://github.com/jackaudio/jack-example-tools/blob/767acf0c93cbcccefd4d9ad89bb6779caffcf2b4/tools/load_test.c#L49

which calls jack_get_time()

https://jackaudio.org/api/group__TimeFunctions.html#ga3b26460e62a56cf012e9075dcb1b6294

but jack_get_time returns microseconds. This PR just updates the help text to describe the current behaviour.

[fps]

Note: the build failure in CI is not caused by this PR