Correct description for kern.proc.proc_td

kern.proc.proc_td returns the process table with an entry for each
thread. Previously the description included "no threads", presumably
a cut-and-pasteo in 2648efa621748.

Description suggested by PauAmma.

PR: 253146
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation

(cherry picked from commit edc374e7c41d3d9e28e0b3a76bc9ed021d7db571)
(cherry picked from commit dbe043d42cda38e7f4186f0c0ed39569b713f05b)