doc: update openssl commands to use best practices
doc: update openssl commands to use best practices

This updates key size to 2048 and default hash function to sha256.

Reviewed-by: Fedor Indutny <fedor@indutny.com>
PR-URL: https://github.com/joyent/node/pull/8690

Cherry-picked-from: https://github.com/joyent/node/commit/88bd95cfef5973de0027b8eb5210e5e97252c7e2

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