docs: admin-guide: Fix min value of threads-max in kernel.rst

Since following patch was merged 5.4-rc3, minimum value for
threads-max changed to 1.

kernel/sysctl.c: do not override max_threads provided by userspace
b0f53dbc

Fixes: b0f53dbc ("kernel/sysctl.c: do not override max_threads provided by userspace")
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>