docs: pdfdocs: Add space for chapter counts >= 100 in TOC

docs: pdfdocs: Add space for chapter counts >= 100 in TOC

It turns out that networking.pdf has exceeded 100 chapters and
titles of chapters >= 100 collide with their counts in its table
of contents (TOC).

Increase relevant params by 0.6em in the preamble to avoid such
ugly collisions.

While at it, fix a typo in comment (subsection).

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/bdb60ba3-7813-47d0-74f9-7c31dd912d95@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>