pmcstat(8): Drop .Ud

This utility has existed for a long time and should not be advertised as
"currently under development".

While here, fix the one other warning from igor about using a newline
for a new sentence.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit a5b3e56330b07c8163b4d62ec55edf37746c11cf)