Fix small but important regression

In OpenSSL pre 1.1.0, 'openssl x509 -CAkeyformat engine' was possible
and supported.  In 1.1.0, a small typo ('F' instead of 'f') removed
that possibility.  This restores the pre 1.1.0 behavior.

Fixes #4366

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4643)