init/Kconfig: Fix CPU number in LOG_CPU_MAX_BUF_SHIFT description

Currently, LOG_BUF_SHIFT defaults to 17, which is 2 ^ 17 bytes = 128 KB,
and LOG_CPU_MAX_BUF_SHIFT defaults to 12, which is 2 ^ 12 bytes = 4 KB.

Half of 128 KB is 64 KB, so more than 16 CPUs are required for the value
to be used, as then the sum of contributions is greater than 64 KB for
the first time. My guess is, that the description was written with the
configuration values used in the SUSE in mind.

Fixes: 23b2899f ("printk: allow increasing the ring buffer depending on the number of CPUs")
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200811092924.6256-1-pmenzel@molgen.mpg.de