Clarify the configuration module in config.pod

Similar to 0652e8a7 ("Clarify default section in config.pod",
2018-04-12), reword a sentence to make it easier to parse.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5794)