Fix example in crl(1) man page

The default input format is PEM, so explicit "-inform DER" is needed to
read DER-encoded CRL.

CLA: trivial

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7094)