[gdb/tdep] Fix doc string of kvm pcb/proc command

On x86_64-freebsd, I ran into:
...
$ gdb -q -batch -ex "maint selftest help_doc_invariants"
Running selftest help_doc_invariants.
help doc broken invariant: command 'kvm pcb' help doc first line is not \
  terminated with a '.' character
Self test failed: self-test failed at command-def-selftests.c:120

Failures:
  help_doc_invariants

Ran 1 unit tests, 1 failed
...

Fix this by adding the missing terminating dot.

Likewise for the kvm proc command.

Tested on x86_64-freebsd.