--progress output refers to cores rather than threads

[Britton Kerin]

On my system:

$ parallel --number-of-threads
20
$ parallel --number-of-cores
14
$ parallel --progress "sleep" ::: 1 1

Computers / CPU cores / Max jobs to run
1:local / 20 / 20

Computer:jobs running/jobs completed/%of started jobs/Average seconds
to complete
local:0/2/100%/0.5s

So the output says "cores" when it should say "threads".  The
cores/threads distinction is discussed in the documentation, but I'd
still say "CPU threads" to be clearer.

Britton