ZMTP-PLAIN: prose contradicts ABNF

[cite-reader]

The prose contains this language:

All command bodies consist of an 8-character command name, padded with spaces, [...]

This contradicts the ABNF immediately prior, in which command names are of variable length and unpadded.

Libzmq's implementation conforms to the ABNF, so it's probably the prose that should be updated.

[bluca]

Feel free to send a PR to fix it