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>

Documentation/admin-guide/sysctl/kernel.rst

@@ -1103,7 +1103,7 @@ During initialization the kernel sets this value such that even if the
 maximum number of threads is created, the thread structures occupy only
 a part (1/8th) of the available RAM pages.
 
-The minimum value that can be written to threads-max is 20.
+The minimum value that can be written to threads-max is 1.
 
 The maximum value that can be written to threads-max is given by the
 constant FUTEX_TID_MASK (0x3fffffff).