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
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit edc374e7c41d3d9e28e0b3a76bc9ed021d7db571)