stats.3: Fix "skipping end of block" syntax errors

These were reported by mandoc -T lint ... as errors.

The rendered output (in ascii and html) is not affected by this commit.

There was a small misunderstanding about the mdoc format: a 1-line
enclosure does not need a closing tag. For example, Pc is for closing
an Po; if a line uses Pq (enclose the remainder of this line), there's
no need for Pc.

Signed-off-by: Graham Percival <gperciva@tarsnap.com>
Reviewed by: mhorne
MFC after: 3 days
Sponsored by: Tarsnap Backup Inc.
Pull Request: https://github.com/freebsd/freebsd-src/pull/1434