Improve readability of evp.pod

The changes are analogous to the ones made in commit 0bf340e1350e
to x509.pod, see PR #4924.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/5012)
