mgmt/mcumgr: Kconfig: Correct OS_MGMT_TASKSTAT_THREAD_NAME_LEN info

The help for the OS_MGMT_TASKSTAT_THREAD_NAME_LEN stated that
the config includes terminating zero, but this is not true:
when thread name is encoded with CBOR it is encoded with actual
string length and is not zero terminated.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>