openssl-dgst.pod.in: Fix documentation of -list option

openssl-dgst.pod.in: Fix documentation of -list option

Mention openssl list -digest-algorithms, NOT -digest-commands.

Move option -list just after the related option -digest.

Fix HTML formatting of section 'Examples' by adding missing
newlines and add 2 examples variant to clarify syntax of the
command.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17626)

(cherry picked from commit 5719dd461fc2cc5d5d29fc3d7e9a6deca3130a7e)