Update --keyfile documentation; remove reference to Base64

[daethnir]

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The --keyfile expects a file containing a chiselKey (ECDSA key starting with "ck-") or the chiselKey itself as an argument.

Documentation indicated you could supplying it a base64 encoded version of chiselKey, but that's not what the code expects today at least.

[daethnir]

Would fix https://github.com/jpillora/chisel/issues/498