Update manual about the default value of `-T#`/`--threads=#`

Update manual about the default value of `-T#`/`--threads=#`

The section about `ZSTD_NBTHREADS` already explains the default number of
threads, since it changed from 1 (commit 17beeb5).  But the option description
for `-T#`/`--threads=#` incorrectly said the default was still 1.

I noticed this when I found compression slower with `-T1` than without it.