Clarify default section in config.pod

This is a minor update which hopefully makes these particular lines
read a little easier.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5938)

(cherry picked from commit 0652e8a7)