Fix typo in keypair subcommand help

[docteurklein]

"public" and "private" where inverted in the keypair subcommand:

        --only-private-key
            Only output the public key

        --only-public-key
            Only output the private key

[divarvel]

Thanks! Before I can merge the PR, you'll need to sign the eclipse eca (CONTRIBUTING.md explains how)

[docteurklein]

oops sorry! I approved the ECA and triggered a rebuild.